

1878.] PROF. OWEN ON THE SHELLS OV CEPHALOPODS. 955 



6. On the Relative Positions to their Constructors of the 

 Chambered Shells of Cephalopods. By Prof Owen C B 

 F.R.S., F.Z.S., &c. 



[Keceived October 4, 1878.] 



(Plate LX.) 



The relations of the chambered and siphonated shells to their con- 

 structors can now, owing to the extent to which they have been 

 subject to extinction, only be fully elucidated by a study of species 

 or varieties of two of the genera, Nautilus and Spirula. 



The fossil shells of Cephalopods, as is well known, exhibit a pro- 

 gressive uncoiling from Nautilus to Orthoceras and from Ammonites 

 to Baculites, with various modifications of the process ; but that this 

 had been carried out to a coiling in the reverse direction, required 

 anatomical evidence for its demonstration. 



The insight, however, gained into the organization of Spirula 

 peronu at the date of the publication of the * Zoology of the Voyage 

 of the Samaraug' 1 , added to that previously obtained on the organi-' 

 zation of Nautilus pompilius', led me to express a conviction of their 

 shell-relations in the following terms :— " These shells (Nautilus and 

 Ammonites) are revolutely spiral or coiled over the back of the 

 animal, not involute like the Spirula "\ And, if the direction of the 

 coils be determined by their relation to the back and belly of the 

 framer of the shell, no other interpretation can be given of such rela- 

 tion as it is exhibited in Spirula (Plate LX. fig. 4) and Nautilus 

 (lb. fig. 3) respectively. 



It is, however, to the exposition and characters of the extraordinary 

 number and manifold variety of the extinct Cephalopods, now known 

 only by their polythalamous and siphonated shells, that an exact 

 and accepted determination of their relative position to the body of 

 the framer is most needed. 



In the year 1829 Leopold von Buch initiated, in his notable 

 memoirs on the Ammonites 4 , the definition and nomenclature of the 

 shell-characters by which the species, genera, and families might be 

 defined. 



Assuming, and correctly in my judgment, that the shells of an 

 Ammonite and a Nautilus were coiled in the same direction, he 

 premises : — " Le caractere distinctif entre ces deux genres de Cepha- 

 lopodes, consiste en ce que le syphon des Ammonites est toujours 

 dorsal, et qui'il ne l'est jamais dans les Nautiles " s . Next, calling 

 attention to the lobed and foliaceous sutures of the Ammonitic shells, 

 he defines the parts which he calls "lobes" and "selles" (saddles)— 

 specifying of the former, " le lobe dorsal," " le lobe ventral;' and "les 

 lobes lateraux" 6 . Von Buch's descriptions and figures admit of no 



1 "Mollusca," Part 1, 4to, p. 6, pi. iv. 



2 Memoir on the Pearly Nautilus, &c. 4to, 1832. 



3 Paleontology, 8vo, 1861, p. 97. 



* Annalescles Sciences Naturelles, 8vo, tome xvii. p. 267, tome xviii. p 417 

 lb. p. 268. e Ib io 1 



