lii 
SYNOPSIS OF GENERA. 
Protomya, n. gen. (Palacomya by error, in Explanation of Plate lxiii; 
not Palseomya, Zittel and Goulers). [Type, Protomya oblonga, Hall.] 
Shell equivalve, inequilateral, elongate, ovate-elliptical. Anterior end broadly 
rounded. Posterior end narrower and regularly rounded at the extremity. 
Beaks incurved. Umbo prominent. Cardinal line long, nearly straight. Um- 
bonal slope gibbous above, merging into the general convexity below. 
Surface marked by fine striae of growth and strong concentric undulations. 
Hinge characters unknown. Ligament external. Muscular impressions cir¬ 
cular, the anterior one strong and situated near the anterior margin. 
This form cannot with propriety be referred to any of the established genera 
of palaeozoic fossils. It differs from Grammysia very distinctly in the absence 
of a mesial cincture, umbonal ridge, and deep ligamental cicatrix; and less 
strongly from Allorisma in the same characters. The beak is more nearly cen¬ 
tral than in either of these genera and the surface presents no evidences of 
radiating striae. 
Promacrus, Meek (Am. Jour. Conchology, vol. vii, p. 4. 1871). 
[Type, Promacrus Andrewsi, Meek.] 
Shell equivalve, sub-equilateral, elongate, sub-trapezoidal; anterior end much 
produced and narrowly rounded at the extremity; posterior end more or less 
produced, obliquely truncate. Beaks sometimes sub-central, appressed, rising 
little above the cardinal line. Cardinal margin nearly straight behind the beaks, 
and declining in front. Umbonal slope distinctly angular, extending to the 
post-basal extremity and leaving a broad cardinal slope above. 
Surface marked by fine strife of growth which are often fascicled and raised 
into more or less distinct undulations. The anterior portion of the shell is 
marked by strong radiating interrupted strife or ridges, which give the surface 
a cancellated appearance. 
Hinge characters undetermined. There are evidences of teeth beneath the 
beaks, but no lateral teeth have been observed. The ligament is external. 
Muscular impressions unknown. 
Examples: Promacrus Missourimsis , Swallow, sp. 
Promacrus cuneatus, Hall, pi. lxxviii, fig. 28. 
