956 PROF. OWEN ON THE [Nov. 19, 



doubt as to his conclusion 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 my conclu- 

 sions on the latter point were inferential, a mere fragment only of 

 the shell having been left attached to the unique specimen submitted 

 to my scalpel in 1832. 



Accordingly, in 1835, M. de Blainville', adopting Von Buch's 

 view of the " dorsal " position of the siphuncle in Ammonites, and con- 

 ceiving 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 Ammo- 

 nites, with little change of the families into which these fossils had 

 been arranged by Von 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 my 

 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," " kevidorses," &c. have gained currency or accept- 

 ance. 



Alcide d'Orbignv 2 adopts the view of relative position, the ter- 

 minology, 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 Von Buch are retained. Prof. 

 Ansted associates Spirula with Nautilus in his family Nautilacea, 

 and writes : — "The 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, Nautilacea, this important part is some- 

 times 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 Ammoneata, 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 Ammonitidae 

 was ventral, as in Spirula, would have implied that the shells were 

 coiled in reverse directions — an assumption seemingly held to be too 



1 Prodrome d'une Monographic des Ammonites, 8vo, 1840. 



2 Paleontologie Francaise, 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," &e. 



3 " On some new Species of the Genus Anct/loceras" ("The ribs ornamented with 

 two conical tubercules on the dorsal part''), Ann. & Mag. of Nat, History, 1845, 

 vol. xv. p. 32, pi. vi. figs. 3, a, d. 



1 Observations on the Animals inhabiting multilocular Shells, 8vo. 

 5 lb. p. 278. 



