152 ANATINID^. 



Pholadomya was largely represented in the Jurassic, decreased 

 considerably in the tertiary period, and is now nearly extinct, 



CYMELLA, Meek, 1864. Shell small, subequilateral, ovate, with 

 numerous well-defined concentric undulations, crossed on the 

 middle of the valves by a few impressed lines, not marked in the 

 depression between the ridges, P. undata^ M. and H. Creta- 

 ceous ; Texas. 



LIOPISTHA, Meek. Shell transversely subovate, ornamented, 

 excepting on the posterior dorsal portions of the valves, by 

 regular, simple, well-defined, sometimes subcrenate, radiating 

 costse, P. elegantula, Romer, Cretaceous ; Texas. P. frequens, 

 Zitt. (cix, SI). 



PSiLOMYA, Meek. Radiating striae or ridges nearly or qui^ 

 obsolete, the radiating rows of granules or spines, usually more 

 distinct ; sometimes with well-defined concentric furrows and 

 ridges. P. superba, Stoliczka. 



MACHOMYA, Loriol, 1868. Shell oblong, subcompressed, equi- 

 valve, strongly inequilateral, rather solid and with punctated 

 surface ; a strong, radiating rib issues from the umbones, and 

 runs towards the anterior margin ; ligament external, solid. 

 The hinge is not known, but the form of the shell, with its 

 strong external ligament, appears to form a passage to the 

 Panopsea. M. Dunkeri, Orb. Jurassic. 



MARGARiTARiA, Conrad, 1862. Its peculiar muscular and 

 pallial impressions should perhaps rank it as a genus. It has 

 not been characterized. P. abrupta, Conrad. Miocene ; Atlantic 

 Slope, United States. 



? ACTiNOMYA, Ch. Maj'cr, Appears to = Margaritaria. 



Palanatina, Hall, 1869. 



Distr. — P. typa, Hall. Fossil. Chemung Gr., New York. 



Shell transversely elongate, very inequilateral, inequivalve, 

 and gaping at the extremities ; left valve the larger and most 

 convex, with a subangular umbonal ridge, and a broad, shallow, 

 anteromesial constriction passing obliquely from beak to base ; 

 the beak small and prominent ; right valve much less convex, 

 with a smaller beak and faint umbonal ridge and sinus ; valves 

 united by a small external ligament ; hinge without lateral teeth, 

 but provided with a small, hook-like process in each valve, just 

 anterior to the beaks, which may have served the purpose of 

 teeth, or more probably for the support of an internal cartilage ; 

 muscular impression very small and indistinct ; the anterior scar 

 rounded, situated just in front of the beak, and near the cardinal 

 margin ; the posterior scar somewhat larger, situated about one- 

 third the distance from the beaks to the posterior extremity ; 

 pallial line not satisfactorily determined, but no evidence of a 

 sinus has been detected ; exterior surface marked b}^ fine con- 

 centric striae. 



