xlii SYNOPSIS OF GENERA. 



Surface marked by concentric striae of growth, and by radiating striae which 

 may be partially or entirely obsolete. Ligament external, small. Interior 

 characters of hinge and teeth undetermined. Muscular impressions sliallow. 

 Prtllial line undetermined. 



This generic name has been proposed for a fossil species of the Hamilton 

 group, described by Mr. Conrad as Nuculites subemarginata (Journal of the 

 Academy of Natural Sciences, Phila., vol. viii, p. 249 ; pi. xv, fig. 5. 1842). 

 It is very distinct in external characters from any form of Nuculites in the 

 absence of the clavicle, which is usually shown near the anterior end, and 

 there is no evidence that the hinge is furnished with teeth of any kind. 



The general expression of the fossil suggests its relationship to the Tellinidae. 



Example: Tellinopm subemarginata, plate Ixxvi, figs. 21-31. 



Cimitaria, Hall (Prelim. Notice Lamellibranchiata, 2, p. 66. 1869-70). 

 [Type, Cypricardites recurva, Conrad.] 



Shell equivalve, extremely inequilateral, transversely elongate ; form solenoid, 

 falcate or elongate-trapezoid. Anterior end short and rounded. Posterior end 

 elongate, the extremity truncate. Beaks strongly incurved, umbones promi- 

 nent. ' Cardinal line straight or concave. Umbonal slope angular and often 

 strongly defined. 



Surface marked by strong concentric lines of growth, which are simple or 

 lamellose, and by fine radiating strias, which may be more or less strongly 

 marked or altogether obsolete ; a more or less distinct depression or cincture 

 extends from the umbones in a slightly posterior direction to the basal margin. 



The hinge is marked by a narrow, elongate, ligamental groove, and anterior 

 to the beaks is a strongly defined lunule. Ligament external. Teeth unknown. 

 Muscular impressions and pallial line undetermined. 



The species placed under this genus are remarkable for their transversely 

 elongate forms, the greater portion of which is posterior to the beaks. Some 

 of the forms bear a resemblance to Orthonota, but the hinge-line shows a dis- 

 tinct ligamental groove and the anterior margin is rounded, with a well-defined 



