lu SYNOPSIS OF GENERA. 



Protomya, n. gen. (Palaeomya by error, in Explanation of Plate Ixiii ; 

 not Paloeomya, 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 Alhrisma 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. 1^71). 



[Type, Promacrus Andrews!, 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 broiid cardinal slope above. 



Surface marked by fine striae 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 striae or ridges, which give the surface 

 a cancellated appearance. 



Hinge characters undetermined. There are evidences of teeth beneath the 

 beaks, but no hateral teeth have been observed. The ligament is external. 

 Muscular impressions unknown. 



Examples : Promacrus Missourimsis, Swallow, sp. 



Promacrus cuneatus, Hall, pi. Ixxviii, fig. 28. 



