18/8.] SHELLS OF CEI'HALOPODS. 965 



Accordingly, the dorsal wall, A, of the spiral shell of Spirilla 

 describes a convex curve, the ventral wall, B', a concave curve. In 

 Nautilus the curves are reversed. If the shell of Orthoceras or 

 Baculites were curved so as to present the same relative positions 

 of convexity and concavity which exist in that of Spirula, they must 

 have been coiled in a reverse direction to that which is presented in 

 the shells of Nautilus and Ammonites. 



Admitting that the siphon in Ammonites and Spirula are both 

 "ventral" or "ventro-marginal," yet the relative position to the 

 shell is so contrasted as to call imperatively for terms indicative of 

 such contrast. And such terms are obvious. Of a convolute shell 

 the convex curve is the outer one, the concave curve the inner one, 

 whatever may be the relative position of its constructor. 



Had Yon Buch been content to call those curves in a convolute 

 Ammonite "external" and "internal," and to define the position of 

 the siphon as "externo-marginal" and "interno-marginal" respec- 

 tively, his terminology would have stood and been unquestioned to 

 the present day. But he proceeded to a conclusion as to their 

 relations to the constructors of the shells ; and, as no specimen of 

 either a Nautilus or a Spirula had been dissected in 1829, such 

 conclusion could only be a guess. 



So probable, however, seemed the guess, that his most experienced 

 contemporaries, De Blainville, Grant, and Gray, after the organiza- 

 tion of the Pearly Nautilus had been made known, preferred, as we 

 have seen, the opinion of the conchologist to that of the anatomist, 

 and deemed the latter to have reversed the true position of the animal 

 of the Nautilus 1 . And, with most 2 , the nomenclature of the parts of 

 the shell of the Ammonitidce. has continued in concordance with that 

 opinion to the present day. 



If, however, the facts and inferences now submitted to the 

 Zoological Society should be accepted with their logical applica- 

 tions in conchology, and the siphon in both Ammonites and Spirula 

 be acknowledged to be " ventral," or " ventro-marginal," nevertheless 

 the different positions of the siphon in these shells and in that of 

 Nautilus demand to be defined by distinctive terms. And these 

 are easy, obvious, and incontrovertibly applicable. The siphon in the 

 Ammonitidce is "external," "externo-marginal," or " ecto-marginal." 

 The siphon in Spirula is "internal," "interno-marginal," or " ento- 

 marginal." The siphon in the Nautilidce may be "central," or 

 "subcentral;" and if the latter, either "ectocentral" or "entoceutral," 

 according as it deviates from the typical central position toward the 



1 See notes 2 and 3, p. 957. 



2 Even in S. P. Woodward's classical ' Manual of Mollusca,' 12mo, " dorsal " 

 and "ventral," are used as synonyms of " external" and "internal," Thus the 

 genus Ammonites is characterized by "siphuncle dorsal" (p. 94, and p. 197, ed. 

 1868). The genus Goniatites is characterized by " siphuncle dorsal " (p. 93). 

 The experienced Editor, in a later edition of the ' Manual of the Mollusca,' in 

 his ' Appendix ' (12mo, 1868) approximates to the conclusion here advocated, by 

 amending the character of Fani. iii. — Ammonitidce,, thus: — " Siphuncle — convexo- 

 marginal ? " (p. 10). If the present paper should serve to dissipate Mr. Tate's 

 remaining doubt it will be an acceptable return for my labour. 



Proc. Zool. Soc— 1878, No. LXIII. 63 



