On Cretaceous Cephalopoda from Z/ihi//m/L 253 



third layer is very thin between the ribs, but apparently continuous 

 with the material deposited on the rib-bases and forming compara- 

 tively high and very sharp ridges, and the whole is covered by a 

 fourth outer layer of shell. Where the acute costation is worn off, 

 the septate rib-bases show as illustrated in Wood's fig. 5 a. 



Woods compared the form with Stoliczka's Heteroceras indicuni, 

 which is considered by various writers to be identical with, or closely 

 allied to, Boxtrychoceras polyplocum, Eomer sp., but which, like 

 Turrilites saxonicus, Schliiter, may be a Hij^hantoceras, not a Bostry- 

 choceras. On the other hand, the small constricted specimen figured 

 as Helicoceras indicum ? by Anderson* and which is not identical with 

 the Indian species, shows open coiling similar to the South African 

 form, but it also may be a Hyphantoceras. 



Of the many forms included in Bostrychoceras pulyplocum (Eomer) 

 by Schliiter,! that figured on pi. xxxv, fig. 8, or the evolute form 

 figured by Geinit^+ may belong to species allied to the South African 

 examples. B. ? dec-live, Gabb sp., B.japoniciun, Yabe sp.,j| further, the 

 whorl-fragments figured by Jimboll as " Tiirrilites sp." and by Meek** 



* " Cret. Depos. of the Pacific Coast," 'Proc. Calif. Acad. Sci.,' 3rd ser., Geol. 

 II, i, p. 91, pi. iii, figs. 9(5 and 97. 



t A specimen of a Bostrychoceras sp. n., iu the Hritish Museum (No. 74042) 

 from Halclem in Westphalia, comparable with Schliiter's pi. xxxiii, figs. and 

 8, from the same locality (Heteroceras polyplocum (A. Romer) in Schliiter) has a 

 costate early portion, then a constriction, and a bituberculate final portion. 

 This form and, perhaps, more so Schliiter's fig. 1 of pi. xxxiv, then Bostry- 

 choceras ? carlottensis, Whiteaves sp. (loc. cit., p. 271, pi. xxxiv, fig. 1) and 

 B. ? oshimai, Yabe sp. (loc. cit., 1904, p. 12, pi. iii, figs. 5 and 6, which, however, 

 may be a Hyphantoceras), show close resemblance in ornamentation to certain 

 Didymoceras, e.g. D. nebrascense, Meek sp. (E.M. No. 83907 from Upper Missouri), 

 and D. hornbycnsc, Whiteaves (B.M., Hector Collection from Vancouver Island). 

 The separation of fragmentary examples of the two genera Bostrychoceras and 

 Didymoceras, based only on tubereulation and mode of coiling, seems somewhat 

 artificial, as a comparison of such typical forms of Didymoceras (in Hyatt) as 

 D. cochleatum and D. tortum, Meek sp., with some of the tuberculate varieties 

 of B. polyplocum in Schliiter will demonstrate. The suture-lines, also, are of a 

 similar pattern in the whole family Nostoceratidae. 



I " Das Elbthal-Geb. i. Sachsen," II, ' Palseontogr.,' vol. xx (1872-5), p. 195, 

 pi. xxxvi, fig. 3 only. (" Turrilites poiyplocus var. of Helicoceras type.") 



' Pal. of California,' vol. i, p. 73, pi. xxviii, figs. 200, 200 a. Upper Chico in 

 Anderson, loc. cit. (1902), p. 27, Maestrichtian in Hang, ' Traite,' p. 1347. 



|| "Cret. Ceph. from the Hokkaido," ' Jl. Coll. Sci. Imp. Univ. Tokyo,' vol. xx 

 (1904), p. 17, pi. iii, fig. 8. 



If "Beitr. z. Kenntn. d. Kreidef. v. Hokkaido," 'Pal. Abli./ vol. vi (1894), 

 p. 41, pi. i, fig. 8. 



*'* Loc. cit. (1876), pi. xxi, fig. 4. 



