52 ANATINIDA. 
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. /. undata, M. and H. Creta- 
ceous ; Texas. 
LiopistHA, Meek. Shell transversely subovate, ornamented, 
excepting on the posterior dorsal portions of the valves, by 
reoular, simple, well-defined, sometimes subcrenate, radiating 
coste. P. elegantula, Romer. Cretaceous; Texas. P. frequens, 
Zitt. (cix, 87). 
pstnomMyA, Meek. Radiating striz or ridges nearly or quite 
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, subeompressed, equi- 
valve, strongly inequilateral, rather solid and with punctated 
surface e; 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 
Panopzea. 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. Mayer. 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 by fine con- 
centric striz. 
