ORDOVICIAN FAUNAS. 131 
direction. All these species agree in having the broad, open umbilicus, 
and it is possible that all should be included in a single genus, which 
would, in that case, include both sinistral and dextral shells. 
PLATYCERAS ? COLUMBIANA MN. sp. 
Plate IV.,, Figs. t-8. 
Description.—Shell laterally compressed, composed of about two. 
volutions ; apex small. closely incurved, slightly depressed below the 
general plane of the shell. Outer volution expanding rapidly, sharply 
carinate along the dorsum; one side gently convex and but slightly 
elevated above the apex of the shell; the opposite side much more 
convex and rounding into a rather broad, open umbilicus. Surface 
of shell not well preserved, but apparently smooth, save for some 
inconspicuous lines of growth. 
The dimensions of the type specimen are: maximum diameter, 13.5 
mm.; length of aperture, 12 mm.; width of same, about 5 mm. 
Remarks.—Only two specimens of this species, exhibiting opposite 
sides of the shell, have been observed, both of which have one side 
buried in the matrix. There is some question as to the genus in 
which this species should be placed. In a general way it resembles 
some species of Raphistoma or of Maclurea, but it differs from any 
of these in its small number of volutions, the outer one of which 
expands rapidly. It does not altogether agree with members of the 
genus Platyceras, but it may be provisionally considered as a much- 
compressed species of this genus. 
CEPHALOPODA. 
CYRTOCERAS sp. undet. 
Plate III., Figs. 7-8. 
Only two specimens of cephalopods have been observed in the fauna. 
One is a weathered, longitudinal section, exhibiting the chamber of 
habitation and a little more than 10 mm. of the septate portion oi 
the shell. The other is an imperfect fragment of a shell about 8 
