MUSEUM OF COMPARATIVE ZOOLOGY. 101 



manner, and attributes the spaces between them to the suddenness with 

 which the animal arose from one resting-place to another, which did 

 not permit the secreting surface to fill up the entire tube. Whether 

 this is the true explanation or not does not here signify, since the 

 main points of structure are the same, and the siphon is closed, accord- 

 ing to both authorities. 



Barrande has also, in his immortal work on the Silurian Cephalopoda 

 of Bohemia, given, with his customary fulness and accuracy, a com- 

 plete analysis of the elements of form and structure among the Nau- 

 tiloids. He has, however, settled upon Ascoceras as the prototype, 

 regarding the Vaginati as the nearest allies of Ascoceras. In describ- 

 ing the structure of Ascoceras, this author acknowledges the existence 

 in the young of simple concave shallow septa, pierced by a true siphon, 

 which opens into the bottom of the living chamber, as usual in all the 

 Tetrabranchiata. 



On one side of the living chamber a series of septa are built up, 

 whose sutures reach only partially around its circumference, and the 

 septa themselves in the interior are equally incomplete. The imper- 

 fect septal chambers thus formed do not open into the living chamber, 

 but are closed by the bent edges of the septa, whose free internal 

 borders bend posteriorly until they come in contact with each other. 

 These posterior prolongations are, in Barrande's opinion, the equivalents 

 of the siphonal funnels of the Vaginati, many of which group have 

 siphons open on one side, and constructed not Unlike this large posterior 

 prolongation of the living chamber in Ascoceras. There would be not 

 the slightest hesitation in accepting this opinion, if it were not for the 

 siphon and perfect septa existing in the young. This, according to the 

 laws of development, as they are now understood, would constitute the 

 Ascoceras a degraded type, one which like the Cirripeds among 

 Crustacea, had developed into a structure simpler and of a lower 

 zoological rank, than if the growth had been arrested at a compara- 

 tively early period. This would meet one of Barrande's principal, 

 reasons for considering Ascoceras as lower than the Orthoceratites, 

 which is, that it certainly could not be placed above them, and really 

 possessed, in its immense, incomplete, adult siphon, or posterior pro- 

 longation of the living chamber just described, a much simpler struc- 

 ture than any of the other Nautiloids. There is probably but little 

 doubt that the Ascoceratites, as claimed by Barrande, have all the 



