The Commonwealth prosecuted the defendant on two complaints charging possession of obscene magazines with intent to distribute them in violation of G. L. c. 272, § 29. The defendant filed motions to dismiss the complaints, alleging that the matter possessed constituted books, not magazines, so that the Commonwealth’s actions pursuant to § 29 were premature. General Laws c. 272, § 281, requires as a condition precedent to proceed*367ings under § 29, involving “books,” the in rem procedures set forth in G. L. c. 272, §§ 28C, 28D, 28E, 28G, 28H.1 After denial of these motions the defendant successfully petitioned, pursuant to G. L. c. 211, §§ 3, 4A, for transfer of the cases to this court. A single justice of this court ordered the cases reserved and reported to the full court for decision.
In August, 1974, two Boston police department detectives entered premises controlled by the defendant looking for obscene materials. They purchased two printed publications, each consisting of more than forty pages, bound by staples, containing a series of photographs with incidental text but no advertising or variety in subject matter.2 The publications do not identify their photographers, editors, or publishers. One of them has, on its cover, a volume number I and the “warning[:] if you find sex offensive, do not purchase this magazine.” The publications themselves were offered in evidence.
*368Two witnesses, a retail bookstore manager and a public librarian, agreed that they would classify these publications as paperback photography books, largely because neither publication gave any indication of serialization or periodicity. The witnesses also considered that the publications’ monographic contents, permanent staple bindings, and formats demonstrate that they are books, not magazines. The Commonwealth does not maintain that it proceeded against these publications under §§ 28C-28H before instituting § 29 proceedings.
The defendant argues that these publications are “books” within the ordinary and approved meaning of that word and that this meaning comports with the purposes of G. L. c. 272, §§ 28-32. Because the Commonwealth failed to comply with the requirements of G. L. c. 272, § 281, the defendant maintains the trial judge erred in denying the motions to dismiss. We agree.
We conclude on the evidence offered below that the publications involved herein are books and are not magazines, as matter of law, and therefore the Commonwealth was required to meet the condition of G. L. c. 272, § 281, before instituting § 29 proceedings against dissemination of these publications. Hence, the defendant’s motions to dismiss the complaints must be granted.
1. General Laws c. 272, § 281, states that “[t]he procedures set forth in sections twenty-eight C, twenty-eight D, twenty-eight E, twenty-eight G and twenty-eight H shall be a condition precedent to the institution of any proceedings pursuant to section twenty-nine or thirty for dissemination of obscene books.” Section 28C, as appearing in St. 1974, c. 430, § 3, provides for in rem proceedings against books believed to be obscene, with notice by publication generally and by registered mail “to the publisher ..., to the person holding the copyrights, and to the author, in case the names of any such persons appear upon said book____” Section 28D allows anyone interested in a book’s dissemination to file an answer, while § 28E permits adjudication of obscenity by general default if no *369person answers. Sections 28F, 28G, and 28H deal with the in rem hearing itself and its legal effects.
The statutory scheme does not define “books,” although it defines “matter” as comprising “any printed material... including but not limited to, books, magazines,... pamphlets ____” G. L. c. 272, § 31, as appearing in St. 1974, c. 430, § 12. When a statute does not define its words we give them their usual and accepted meanings, as long as these meanings are consistent with the statutory purpose. See Commonwealth v. Gove, 366 Mass. 351, 354 (1974); Franki Foundation Co. v. State Tax Comm’n, 361 Mass. 614, 617 (1972). We derive the words’ usual and accepted meanings from sources presumably known to the statute’s enactors, such as their use in other legal contexts and dictionary definitions.
The words “books” and “magazines” have established definitions in other legal contexts. In 1912, Mr. Justice Holmes, construing Federal postal statutes, said “generally a printed publication is a book when its contents are complete in themselves, deal with a single subject, betray no need of continuation, and, perhaps, have an appreciable size.” Smith v. Hitchcock, 226 U.S. 53, 59 (1912). A United States District Court cited this language in the context of the Montana libel statute, looking also at the Montana Legislature’s purpose in giving special treatment to books (opportunity to correct libellous matter). Fifield v. American Auto. Ass’n, 262 F. Supp. 253, 255-257 (D. Mont. 1967). Similarly, looking at the legislative purpose in dealing specifically with “magazines,” courts generally have defined “magazines” as a subspecies of periodicals, emphasizing their periodicity, their continuity as to title and nature of contents from issue to issue, and their authorship usually by an editorial staff rather than by a single author. See id. at 256-257 (libel statute); Application of Wings Publication Co., 148 F.2d 214, 215 (C.C.P.A. 1945) (trademark statute); Business Statistics Organizations, Inc. v. Joseph, 299 N.Y. 443, 449 (1949) (sales tax ordinance). These definitions are consistent with the com*370mon understanding of the words at issue as well as the dictionary definitions thereof.3
The apparent legislative purpose in especially protecting books is some indication, as shown in our reasoning below, that the Legislature intended to emphasize periodicity as a distinguishing criterion between books and magazines.
It appears that the Legislature intended that potential defendants under G. L. c. 272, §§ 29, 30, should receive the clearest possible notice of their liability for dissemination of obscene materials, as long as such notice would not compromise other policy considerations of the statutory scheme. Cf. Commonwealth v. 707 Main Corp., 371 Mass. 374, 381-382 (1976). In rem procedures provide disseminators with the clearest possible notice that the materials they disseminate are obscene and therefore dissemination constitutes prohibited conduct.4 See Paris Adult Theatre I v. Slaton, 413 U.S. 49, 54 (1973). These procedures, while providing constitutionally preferable notice, are characteristically expensive and time consuming. The Legislature apparently has concluded that the virtues of in rem notice outweigh the procedure’s time and expense defects when liability for dissemination of “books” is involved, but, when other matter is involved, that the needs for effective en*371forcement and fiscal restraint override the value of in rem notice. Thus, the desirability of unambiguous notice of a book’s obscenity outweighs the risk that the delay inherent in in rem proceedings will result in continued availability for a few months of one obscene publication. On the other hand, the value of constitutionally unnecessary notice may not outweigh the risk that delay for in rem proceedings to suppress an obscene magazine will result in an automatic proliferation of obscene magazines available to the public at low prices through a widespread distribution system. Hence, the element of periodicity which has distinguished books from magazines in other contexts and which had accepted usage prior to the enactment of G. L. c. 272, § 28C-32, may be thought to have direct relevance to the legislative purpose in providing in rem procedures for the adjudication of books’ obscenity.5 6
We conclude that a substantial printed publication is a book for the purposes of G. L. c. 272, §§ 28C-31, if it is complete in itself, betraying no evidence of continuation with publications of a similar nature issued at regular periodic intervals. A motion to dismiss proceedings under § 29 or § 30 must be granted when the defendant proves that the publication disseminated fits the above definition and that the prosecutor failed to follow the procedures set forth in §§28C, 28D, 28E, 28G, and 28H. G. L. c. 272, §281.
2. The defendant in these cases filed timely motions to dismiss alleging that the putatively obscene publications are books and that the requirements of G. L. c. 272, § 281, have not been met. It presented evidence of completeness and lack of periodicity through the testimony of people accustomed to classifying publications by genre and through the lack of indications of continuity or serializa*372tion in the publications themselves. The prosecution presented evidence that the publications have glossy covers, stapled bindings, abbreviated texts unrelated to the photographs which constitute the major content therein, few words, no author or publisher or copyright designation, and, on the cover of one publication, a volume number and the designation “magazine.”
The publications were properly admissible as direct evidence of their own completeness or periodicity. A court may examine publications for evidence of continuity with other publications, and publications themselves can constitute sufficient evidence thereof. In this case, there was no evidence of serialization beyond a volume number on one publication. Since many books are published in several volumes, this fact alone was insufficient for a finding of periodicity. The designation “magazine” constitutes con-clusory opinion testimony by persons unknown applying unknown criteria, if any, and is not probative. In addition, the publications’ formats, while marginally relevant to their appropriate classification, were inconclusive in this case, given the testimony adduced by the defendant that the formats were indicative of paperbound books rather than of periodicals. Although testimony by experts and others who deal routinely with various kinds of publications is not necessary to a decision on publications’ classifications, a judge may properly consider such testimony.
The prosecution argues that even if its evidence does not warrant a finding of periodicity, the evidence shows that the publications do not come within the intended area of § 281 protection. We disagree for the reasons given above. The protection of § 281 (which clearly foresaw the possibility that publications’ authors, publishers, and copyrights would be unknown, see G. L. c. 272, §§ 28C, 28D) should be broadly construed as extending to all matter falling within the classification “books.” The statutory scheme on its face does not confine § 281 protection to marginal cases where notice of obscenity prior to prosecution of disseminators is particularly desirable. See note 4, supra.
*373The defendant has shown, as matter of law, that these publications are books within the meaning of the statutes. Because the prosecution failed to show that it had met the conditions precedent required by G. L. c. 272, § 281,6 the defendant’s motions to dismiss were erroneously denied. Those denials are vacated and the case is remanded to the county court where judgments of dismissal are to be entered as to both complaints.
So ordered.
The word “commencement” is stressed advisedly, since the beginning of in rem proceedings is all that is required prior to indictment. Section 281, inserted by St. 1974, c. 430, §8, says that “[t]he procedures set forth in... shall be a condition precedent to the institution of any proceedings pursuant to ...[§§ 29 or 30] for dissemination of obscene books.” When that section is read in conjunction with § 28H, it is clear that the condition precedent to the commencement of the I 29 or § 30 type case is that the in rem complaint under §§ 28C, 28D, 28E, 28G, 28H be filed, and not that it shall be completed. Section 28H, as appearing in St. 1974, c. 430, § 8, makes this clear by providing that in a prosecution under § 29 for an offense committed after the “filing” of the § 28C proceeding, “the fact of such filing and the action of the court or jury thereon, if any, shall be admissible in evidence.” Section 28H then provides (a) that if the alleged criminal offense occurred after a final decree has been entered “against the book” and the book is obscene the defendant is conclusively presumed to have known that the book was obscene, (b) that if the final decree had been entered “in favor of the book he shall be conclusively presumed not to have known said book to be obscene,” and (c) that “if no final decree had been entered but a proceeding had been filed prior to such offence,... [he] shall be conclusively presumed to have had knowledge of the contents of said book.”
The photographs deal with explicit sexual conduct. However, this fact is not relevant here, since it relates to the obscenity of the publications’ contents, an issue which is not before us.
“[B]ook ... a collection of written, printed, or blank sheets fastened together along one edge ... a long systematic literary composition ....” “[M]agazine... (1): a periodical that usu. contains a miscellaneous collection of articles, stories, poems, and pictures and is directed at the general reading public (2): a periodical... directed at a group having a particular hobby, interest or profession....” Webster’s Third New Int’l Dictionary (1961).
In rem procedures protect potential defendants who disseminate materials they believe in good faith to be nonobscene. Such procedures also protect members of the general public who wish to obtain, without self-censorship by disseminators fearing criminal or civil liability, materials which are not obscene but which are close to the obscenity line. Although ambiguity in the application of a statutory prohibition to marginal cases is insufficient to constitute a violation of the due process clause of the United States Constitution, Miller v. California, 413 U.S. 15, 27 n.10 (1973), see Commonwealth v. 707 Main Corp., 371 Mass. 374, 383 (1976), the law prefers to avoid such ambiguity when possible. Cf. Paris Adult Theatre I v. Slaton, 413 U.S. 49, 54 (1973). Z. Chafee, Government and Mass Communications 228-234 (1947).
Although the prosecution maintains only that these publications are magazines, we note that pamphlets and photographs lack the element of periodicity which distinguishes magazines but still are subjected to the speedier procedure. G. L. c. 272, §§ 281, 31. Possibly, the expense of in rem procedures outweighs the desirability of unambiguous notice when publications of extreme brevity are involved.
As to burden of proof, see the penultimate paragraph in Commonwealth v. Ferro, post, 379, 386 (1977), decided this day.