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Show 956 PROF. OWEN ON THE [Nov. 19, doubt as to hisconclusion that the outer convex curve was the "dorsal " one, the inner concave curve the " ventral " one, in both Nautilus and Ammonites. Such, indeed, seemed the obvious relations to back and belly of the Nautilus-shell before the structure and position of its framer were made known. And it should be remembered that m y conclusions on the latter point were inferential, a mere fragment only of the shell having been left attached to the unique specimen submitted to m y scalpel in 1832. _ Accordingly, in 1835, M . D E B L A I N V I L L E 1 , adopting Von Buch's view of the " dorsal" position of the siphuncle in Ammonites, and conceiving the shell of Spirula to be convoluted in the same direction as in Nautilus, characterizes its siphuncle as " ventral," and that of the Ammonite as "dorsal." In his applications of these views of relative position to the A m m o nites, with little change of the families into which these fossils had been arranged by V o n Buch, he proposes new names for them, and adopts the shape and proportions of the outer curve or border of the shell as the family characters. Thus, when such outer, and in m y view ventral, curve of the shell is broad, as in Ammonites dilatatus, a family of " A. latidorses " is diagnosed ; a reverse proportion, as shown in Ammonites discus, characterizes the family " A. compressi-dorses;" but neither these families, nor those of the " cavidorses," "cristidorses," " lsevidorses," &c. have gained currency or acceptance. A L C I D E D ' O R B I G N Y 2 adopts the view of relative position, the terminology, and in the main the classification proposed by Von Buch. By MORRIS 3 and ANSTED 4 the aspects of the Nautiloid and Ammonitoid shells propounded by V on Buch are retained. Prof. Ansted associates Spirula with Nautilus in his family N A U T I L A C E A , and writes : - " T h e next point of difference to be attended to is in the siphuncle; and it is one both of position and magnitude. In the genera of the first family, N A U T I L A C E A , this important part is sometimes ventral, or on the inner margin, more frequently central, and is very rarely observed to approach the dorsal or outer margin. On the other hand, it is almost always very near the dorsal margin in the A M M O N E A T A , and sometimes is actually placed outside, in a channel opened for it, and projecting from the back of the shell in the shape of a keel "5. In fact, to have propounded that the siphuncle in the Ammonitidse was ventral, as in Spirula, would have implied that the shells were coiled in reverse directions-au assumption seemingly held to be too 1 Prodrome d'une Monographie des Ammonites, 8vo, 1840. 2 Paleontologie Franchise, 8vo, 1842, p. 185:-"Les lettres suivantes, les memes que celles qu'emploie M . de Buch, indiquent toujours les memes parties dans les figures-e. g. D. lobe dorsal, V. lobe ventral," &c. 3 " O n some new Species of the Genus Ancyloceras " (" The ribs ornamented with two conical tubercules on the dorsal part'-), Ann. & Mag. of Nat. History, 1845, vol. xv. p. 32, pl. vi. figs. 3, a, d. J Observations on the Animals inhabiting m unilocular Shells, 8vo. 5 Ib. p. 278. |
