GASTROPODA. 913 
Oxydiscus subacutus.] 
Besides the species listed on page 852 there are three more American, or rather 
Canadian, species that may belong to Oxydiscus. These were described by Billings 
as Bellerophon macer, B. palinurus, and B. argo, the first from the Calciferous forma- 
tion, the second from the Quebec group, the third from the Black River group. Of 
the three forms, the first is the most likely to belong here, but in the absence of 
any positive knowledge respecting their essential generic features, it is best to leave 
them provisionally where they are placed by Mr. Billings. 
Respecting the systematic position of Oxydiscus, we are somewhat in doubt. 
The absence of any sort of revolving lines is against the inclusion of the genus with 
the Bucaniide. On the other hand such a position is strongly indicated by the 
absence of labial callosities and more particularly by the large umbilicus and long 
apertural slit. The latter occurs most certainly in the Lower Silurian species next 
described, and as these species agree verly closely in all respects with the Upper 
Silurian and Devonian types of the genus, we have assumed that the slit is one of 
the prime characteristics of the genus. Now, if this view proves to be correct we 
cannot be far wrong in placing Oxydiscus in the immediate vicinity of Conradella. 
The only difference of any consequence between these genera are that, while the 
surface of the volutions of the latter are crossed by imbricating, wavy lamelle, 
which are not turned backward on the dorsum to form an angular sinus where they 
meet at the keel, the whorls of the former are crossed simply by fine lines of growth, 
turning backward very strongly in nearing the dorsal keel, their junction here 
indicating a narrow, V-shaped excision in the outer lip. 
Oxypisous suBacuTus, . sp. ( Ulrich.) 
PLATE LXII, FIGS, 62-65; PLATE LXXXII, FIGS, 23—25. 
Shell lenticular; dorsum acutely carinated; greatest diameter from 15 mm. to 
28 mm.; greatest thickness or width nearly one-half the diameter. Volutions three 
and one-half to four and one-half, thickest near the umbilicus, from which the 
surface ascends first with a gently convex, then with a concave slope to the sharp 
periphery; each volution embracing between one-third and one-half of the 
preceding one; umbilicus exposing all the whorls; its width somewhat less than 
one-third of the diameter of the shell; edge of umbilicus abrupt, subangular; 
aperture obcordate, indented below by the sharp dorsum of the preceding whorl; 
margin of aperture thin, in a side view with a strong backward sweep; slit long, 
very narrow. Surface marked by fine and rather indistinct lines of growth. 
This species agrees perhaps as well as any with the Devonian types of the 
genus. Of Silurian forms only O. cristatus Safford and O. disculus Billings need be 
—58 
