29 
Ohs. — I am led to propose tliis name for certain of our Cretaceous 
bivalves that cannot he satisfactorily referred to any genus known to me, hut 
which seem to he allied to both Cijrena and Corhicula. They possess the 
general characters of both, except that the cardinal teeth arc two instead of 
three in each valve, whilst the laterals aj^pear to he smooth, as in Cyre/ng and 
not striate, as in Corhicula ; furthermore, the ])allial sinuses are inconspicuous 
as in the former, even if inflections are present at all. 
In the left valve there is a more or less central triangular entire 
cardinal tooth which adapts itself to a large triangular socket in the right 
valve slightly posterior to the umhonal apex. Above and posterior to this 
is the second and narrow oblique cardinal which tits into an oblique socket 
in the right valve. In the latter both cardinals are narrow, oblique, and 
posteriorly directed. 
The adductor impressions in one species are faint, in a second 
moderately strong. 
The number of the cardinal teeth also distinguish it from Cyrenoicles, 
Joannis. There is undeniably a very close external resemblance between the 
shells constituting Cyrenopsis and those referred to Cyrena and Corhicula in 
the Laramie formation of North America, with the exception of the dental 
formula. 
Type. — 3Iactra (vel. Corhicula) Meeki, mihi. 
Cyhenopsis opallites,^ sp. nor. 
(PI. Y, Figs. 12-17.) 
Cyrena (t J, n. SJ),, Giirich, Neues .Tuhrb., 1001, 13eil. Ed., XIV, p. ISG, t. 19, f. 5a, 5b, G. 
Sj). Char. — Shell ovate, modei’ately inflated. Cardinal margins 
arched ; ventral marsins well rounded. Anterior ends somewhat flattened 
with rounded margins ; posterior ends slightly produced or nasutc, the dorsal 
posterior margins straight and oblique, the ventral rounded ; posterior slopes 
slightly flattened, narrow, and generally inconspicuous ; UDibones depressed ; 
escutcheon long, deep, and narrow; ligament short. Articulus arched, longer 
on the posterior than the anterior ; anterior cardinal of the left valve triangular, 
projecting and slightly oblique, posterior cardinal of the same valve laminar ; 
anterior lateral slightly curved, short, posterior lateral, long distant and 
' dirdwioi, the 0}ial, and /trs -- lilhos (contracted), a stone. 
