BEEKMANTOWN AND CHAZY FORMATIONS OF CHAM PLAIN BASIN 429 

 Family P'lLOCER.fas.TIDJ^E 



Genus piloceras Salter 

 Piloceras explanator Whitfield 



Plates 10 and 11 



Piloceras explanator Whitfield. Am. Mus. Nat. Hist. Bui. 1886. 



V. I, no. 8, p. 323, pi. 28, fig. 1-4 

 Piloceras explanator Foord. In Cat. Foss. Ceph. Brit. Mus. pt i. 



1888. p. 162. 

 Piloceras explanator Clarke. Geol. of Minn. Pal. 1897. 3:769, 



pi. 2. 

 Piloceras explanator Ruedemann. An. Rep't State Paleontol. 



for 1903; N. Y. State Mus. Bui. 80. 1905. p. 329, pi. 10-13. 



We have found in the Fort Cassin beds at Valcour (A3 of section) 

 a truly gigantic Piloceras which has proved to be identical with the 

 form which has been so carefully described and fully illustrated by 

 Whitfield as P. explanator from the Fort Cassin beds at their 

 type section. 



In regard to the character of the conch, the cameras, living cham- 

 ber and siphuncle, we refer to Whitfield's description drawn from 

 the excellent material of the Seely collection. The internal struc- 

 ture of the siphuncle of this species which has not been described 

 by Whitfield has been given in detail by the writer in the above cited 

 report of the State Paleontologist, to which we also refer for the 

 sake of avoiding repetition. From the large specimen which is here 

 reproduced in outline, it follows that Whitfield was also correct in 

 assuming that the shells attained at least 10 inches in length, for 

 this one measured as much with the entire living chamber missing. 



This species is probably closely related to P . t r i t o n Billings 

 from the Newfoundland Beekmantown beds and we thought for a 

 time that the two might be identical until we saw the type of 

 Billings's species. 



The dimensions of the siphuncles as well as the distances of the 

 septa agree fully in P. triton, P. explanator and the 

 specimens from Valcour. The elliptic shape of the section of the 

 siphuncle of P. triton, which can be inferred from Billings's 

 measurements furnishes also intrinsic evidence of the lateral com- 

 pression of the entire conch of Billings's type such as is described 

 of P. explanator. If we further take into account that 

 Billings would place his divisions H and I near the boundary of the 

 Beekmantown and Chazy formations, it will be seen that the New- 

 foundland and Lake Champlain forms are found in beds which are 

 equivalent or very nearly so. Billings's type, Avhich is in the 



