MUSEUM OF COMPARATIVE ZOOLOGY. 07 



as may be seen in the ventral view of this part in Goniatites diadema, 

 on Plate IV, where the sutures incline posteriorly at their junction with 

 the ccecurn. Thus, instead of being entirely out of the ovisac as in 

 Nautilus, whether recent or fossil, the siphonal ccecum is to a considerable 

 extent developed within the ovisac. In other words, this part is devel- 

 oped and completed later, as is also the first true septum, of which it is 

 an integral part, in the existing Nautilus, than in the fossil Goniatites, 

 certainly as early as the Carboniferous and perhaps earlier. I should 

 expect, however, to find some change in this respect in the Silurian 

 forms of Goniatites. 



The ccecum is made up of the same elements, if we consider that 

 the internal corneous element has probably been destroyed in the course 

 of fossilization. 



The siphonal funnel at a later age becomes more distinct, and is not 

 confounded with the sheath of the siphon as it is in the first and second 

 septa, and perhaps the third also, but this last fact could not be deter- 

 mined satisfactorily. At a later stage the funnel is very distinctly seen, 

 and, as described by Guido Sandberger, is continuous on the ventral 

 side, though it lies almost its entire length against the shell. Barrande 

 has described this occurring in the same manner in the Goniatites of 

 the Silurian, and thus accounts for the appearance which deceived Von 

 Buch. This author supposed that one of the main distinctions between 

 Goniatites and Nautili lay in different positions of the siphon. He 

 removed the shells of certain Goniatites and Ammonites, and apparently 

 exposed the siphon lying in immediate contact with the shell, whereas 

 he had really torn off the ventral lip of the siphonal funnel with the 

 shell. Barrande, on removing the shells of Goniatites, found that this 

 frequently occurred, and thus established the truth of Guido Sandberg- 

 er's observations. 



The posterior edges of the funnel * rest in and upon the siphonal 

 sheath very nearly as in Nautilus Pompilius, and this sheath, also, as 

 in the latter, connects the siphonal funnels of adjacent septa, as de- 

 scribed in Nautilus. 



The siphonal ccecum of Ammonites possesses very nearly the same 

 form as in Goniatites, but is, in the species examined, perhaps somewhat 

 flatter dorso-abdominally. It has the same position, but. yet no difficulty 

 was experienced in removing the shell without disturbing the ccecum, 



* Plate III, Fijr. 8, P. 



