GEOLOGICAL SURVEY OF CANADA. 



PALEOZOIC FOSSILS. 



VOL. III. 



6. The Canadian species of Plectoceras and Barrandeoceras. 

 By J. F. Whiteaves. 



The genus Plectoceras was first described by Hyatt in 1883, on page 

 268 of his " Genera of Fossil Cephalopoda," published in the twenty-se- 

 cond volume of the Proceedings of the Boston Natural History Society. 

 In that paper it is placed in the Family Tainoceratid*, and is described 

 as follows : — 



"Plectoceeas*, nobis, includes Silurian species having costse curved pos- 

 teriorly on the sides and crossing the abdomen as in Trocholites and 

 sutures similar, but with ventral saddles. The whorls quadrate, the abdo- 

 mens narrower than the dorsum and the sides convergent outwards. The 

 siphons are ventral and holochoanoidal. The young are precisely similar 

 in form, smoothness of the shell and strise of growth, and in sutures to 

 the straight sutured forms of Trocholites. Type, Fleet. ( Naut. ) Jason, sp., 

 Bill., Canad. Nat., vol. 4, 1859, p. 464, Mus. Geol. Surv. Can." 



In the " Phylogeny of an Acquired Characteristic," published in 1894, 

 in the thirty-second volume of the " Proceedings of the American Philo- 

 sophical Society, pages 499 and 500, the genus, and three of its species 

 are thus described. 



" Plectoceras. 



" This genus was described in Oenera of Fossil Cephalopoda, page 268, 

 by the author, to include the costated forms similar to Discoceras, but 

 having the siphuncle ventrad of the centre. 



" The type was Plectoceras (Naut.) Jason, sp. Billings. The mode of 

 coiling may be quite close and regular, with perhaps a slight impressed 

 zone or flattened dorsum, or the coil may be open, and sometimes it is 

 very irregular. In several specimens of Jason the first whorls may touch, 

 the ephebic volution may be open and free and yet the extremity of the 

 living chamber again come in contact. The umbilical perforation is large 



* nXsxTOi, twisted or plaited. 



