482 NEW YORK STATE MUSEUM 



Observations. Owing to the rather sliort generic diagnosis of 

 Deltoceras, which states but certain differences from Aphetoceras 

 and to the lack of any illustrations of the one species representing 

 the genus we have been unable to arrive at a conclusive reference 

 of our species to the latter. At the same time we do not doubt that 

 this species has more nearly attained the plane of development of 

 Deltoceras than of any other genus. It fully agrees with the picture 

 that is drawn of Deltoceras by Hyatt in the rapid growth of the 

 whorls, the loose coiling, the suture^s and the large size and ventral 

 position of the siphuncle, while in the amount of the flatten- 

 ing of the abdomen and its relative great width upon the last whorl 

 (gerontic stage ?) it would seem to differ from or go beyond the 

 conception of the genus. 



Billings has made known three species of Nautilus from the Chazy 

 limestone of the Mingan islands. One of these, Nautilus 

 t y r a n s , appears to have borne some similarity in its habit to our 

 form. This is however not figured and the description being drawn 

 from a single incomplete specimen, is insufficient to permit a 

 definite recognition of that form ; nor has Hyatt reinvestigated or 

 even again mentioned it ; but an inspection of the type 

 specimen of N . t y r a n s in Ottawa has shown us that the volutions 

 are nowhere in contact, that its rate of growth is slower and that it 

 possesses shallower cameras. From all other nautiloids of the Chazy 

 formation D. vaningeni is distinguished by the more rapid ex- 

 pansion of its conch. 



We have three specimens which come from three different beds 

 of the Chazy at Valcour. The type specimen (from bed B^) has 

 retained part of the adult or gerontic living chamber and shows that 

 this became evolute. 



We take great pleasure in naming this stately form after Prof. 

 Gilbert van Ingen, of Princeton University, to whose enthusiastic 

 collecting the State Museum owes so much of its Champiain 

 material. 



Family F»LECXOCEFi.A.TID^^E 



Genus plectoceras Hyatt 



Hyatt has erected this genus [1884, p. 268] to include the cos- 

 tated forms similar to Discoceras [see under Schroederoceras], but 

 having the siphuncle ventrad of the center. 



The original diagnosis of the genus is : 



Plectoceras, nobis, includes Silurian species having costae curved 

 posteriorly on the sides and crossing the abdomen as in Trocholites 



