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The siphuncle is propioventran in the paranepionic and very large ; 

 in subsequent stages it is somewhat nearer the centre and continues 

 to be large. 



Discoceratidce. 



This family includes some genera formerly associated under the 

 Tainoceratidae and others not heretofore noticed in my classifica- 

 tion. 



Although it is a provisional and heterogeneous group in some 

 respects, the genera are alike in being more or less heavily ribbed, 

 and ift having open apertures so far as known. This association 

 also brings together forms having a tendency to develop into more 

 or less turbinate coils. 



The genera are as follows : Peismoceras, Systrophoceras, Trocho- 

 ceras, Mitroceras, Lower Silurian ; Plectoceras and Discoceras, 

 Quebec faunas to Upper Silurian. 



Plectoceras. 



This genus was described in Genera of Fossil Cephalopods , page 

 268, by the author to include the costated forms similar to Disco- 

 ceras, 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 and the impressed zone 

 is absent until the whorls come into contact and it is invariably 

 absent in gerontic whorls. 



The species are as follows : 



Plectoceras Jason, sp. Billings {Canadian Nat., iv, p. 164). 



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 River fauna in New York and is 

 quite commonly mistaken for the young of Eurystomites undatus. 



