330 NEW YORK STATE MUSEUM 



to the existence of types which held the same relation to the 

 phylephebic species of Piloceras as does Proterocameroceras to 

 Cameroceras ; and which would be properly called " Proteropilo- 

 ceras." If in P. e x p 1 a n a t o r the cameras did not extend on 

 one side to near or quite to the apex of this nepionic bulb, we 

 would not hesitate to make this form the type of the proposed 

 subgenus. It is evident that a process of acceleration in the 

 phylogeny of this genus has led to a crowding back of the forma- 

 tion of septa, which originally was the cause of the contraction 

 of the siphuncle, to the very apex of the nepionic bulb without, 

 however, having yet been able to efface all vestiges of this former 

 inflation of the conch. This also points clearly to the process 

 by which the nepionic bulbs of Proterocameroceras and Protero- 

 vaginoceras have become reduced in Cameroceras and Vagino- 

 ceras, i. e. by a tachygenetic encroachment of the metanepionic 

 growth stage on the aseptate ananepionic stage. 



Besides the presence of the nepionic bulb, Piloceras exhibits 

 also in its- endosiphuncular structure characters which link it 

 closer to the Protero-forms of the other associated series, than 

 to Cameroceras. 



The siphuncle is, like the conch, short, conical, with elliptic 

 to oval section [see pl.io] ; the endosiphocone is short and broad 

 with elliptic upper section, rapidly shrinking to a flat blade at its 

 narrower end [see pl.13, Rg.1,2]. Its cast shows peculiar flutings 

 arranged in bundles and which, in one specimen, appear to con- 

 sist of longitudinally arranged pits and strongly remind one of the 

 similar depressed lines found on the outer conch. Since the latter 

 are produced by muscular attachment of the animal within the living 

 chamber, the presence of these scars on the wall of the endosiphocone 

 seems to me a strong argument for the view that in this primitive 

 form the visceral cone shared still to a great measure the functions 

 of the living chamber. We have already seen that in Protero- 

 cameroceras brainerdi a large anterior portion of the 

 siphuncle remained unobstructed by deposits and was evidently 

 occupied by the animal during its lifetime. In Piloceras 

 explanator this portion of the siphuncle was considerably wider 



