499 
The siphuncle is propioventran in the paranepionic and very large ; 
in subsequent stages it is somewhat nearer the centre and continues 
to be large. 
Discoceratide. 
This family includes some genera formerly associated under the 
Tainoceratide 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 in 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 (Vaut.) 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 /ason 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, DN. S. 
This species occurs in the Black River fauna in New York and is 
quite commonly mistaken for the young of Eurystomites undatus, 
