300 



and the impressed zone is absent until the whorls come in contact and it 

 is invariably absent in gerontic whorls. 

 " The species are as follows : 



" Plectoceras Jason, sp. Billings. {Canadian Nat, iv, p. 464). 



" Type in Museum of Geological Survey at Ottawa. It occurs in the 

 Calciferous * of the Mingan Islands and there are similar forms in the 

 same horizon in Newfoundland. 



"Plectoceras obscurum, n. s. 



" This species occurs in the Black Eiver fauna in New York, and is 

 quite commonly mistaken for the young of Eurystomites undatus, but it 

 has an open gyroceran spiral, the siphuncle is nearer the venter and the 

 costse are more highly developed and more prominent, and have a distinct 

 character from those of that species. 



" Plectoceras Bickmoreanus, sp. Whitfield (Bull. Am. Mus., New 



York). 



" This species of the Niagara fauna has an open gyroceran whorl, and 

 in the gerontic stage the last whorl is free and in some specimens com- 

 pletely straightened out and lituitean in aspect." 



Hyatt's latest views on Fossil Cephalopoda are embodied in the article 

 on Cephalopoda in the first volume of Dr. C. E. Eastman's translation of 

 Zittel's Text Book of Paleontology, published in 1900. In this article 

 (p. 520) Phctoceras is made the type of a new family called Plectoceratidse, 

 and this family and the genera of which it is said to be composed are 

 thus described. 



" Family 6. Plectoceratid^. 



" Gyroceracones, nautilicones, and torticones having annular costaefrom 

 the neanic stage until late in life, and in some genera, more or less prom- 

 inent longitudinal ridges, which usually disappear in the ephebic stage. 

 Siphuncle ventrad of centre. 



" Plectoceras, Hyatt. Ordovioian and Silurian, ffphyradoceras, Hyatt 

 (Peismoceras, Systrophoceras, Hyatt). Silurian and Devonian. The first 

 is gyroceraconic, with some discoidal nautilicones, and the second is almost 

 exclusively torticonic of the trochoceran type." 



There appear to be at least three species of Plectoceras in Canada, as 

 follows, though the generic position of the one last cited is still uncertain. 



*Mr. Billings, however, describes Naulilua Jason as a fossil of the "Chazy lime- 

 stone." J. r. w. 



