148 ANATINIDA. 
upper margin no trace of a special furrow, and the beaks are 
closely approximate to each other. 
GressLyA, Agassiz, 1842. 
Distr.—50 sp. Lias, Jura; Europe. G. zonata, Agass. (cvii, 43). 
Shell oval, rather compressed; umbones anterior, incurved, 
not prominent ; valves thin, close, smooth or concentrically fur- 
rowed; pallial sinus deep. The lamina within the posterior 
hinge-margin of the right valve produces a furrow in the casts, 
which are more common than specimens retaining the shell. 
ALLoRISMA, King, 1844. 
Syn.—Cercomyopsis, Meek. 
Distr.—Devon., Carboniferous. A. sulcata, Flem. (evii, 44). 
Shell transversely elongated, very thin, elliptical, equivalve, 
very inequilateral; each ‘valve with an elongated ligamentary 
support; hinge without teeth; valves concentrically plicate, in 
some species radiately punctate, in others the very small puncta- 
tions are distributed without order. 
Myacrres (Schlotheim), Bronn. 
Distr.—Fossil, 50 sp. Triassic—Lower Chalk; United States, 
Europe, South Africa. 
Shell oblong, ventricose, gaping, thin, often concentrically 
furrowed; umbones anterior; surface granulated; ligament 
external; hinge with an obscure tooth or edentulous ; muscular 
impressions faint ; pallial line deeply sinuated. 
PACHYMYA, Sowb., 1826. Shell transversely elongated, modio- 
liform, equivalve, thick; beaks subterminal. Cretaceous. 
HoMOoMYA, Agassiz, 1842. Shell very thin, transverse, oval, 
ventricose, inequilateral, gaping at the extremities; hinge 
without teeth; beaks thick, rounded, a little curved. Jurassic. 
PLEUROMYA, Agassiz, 1842. (Anoplomya, Krauss.) Shell 
elongated or ovoid, very thin, nearly papyraceous, concentrically 
furrowed ; beaks large, swollen, anterior, recurved in front, con- 
tiguous ; pallial impression with a large, profound sinus, rarely 
distinet. D’ Orbigny has found cardinal teeth in well- preserved 
specimens, and refers Pleuromya to Panopxa. Several sp. 
Liassic and Jurassic P. tenuistriata, Agass. 
myopsis, Agassiz, 1845. Shell moderate or large sized, covered 
with linear points forming radiating lines, more or less gaping 
at both extremities ; a car rdinal tooth in each valve; beaks more 
or less anterior, sometimes even marginal; calla impression 
with a a profound sinus ; muscular impressions indistinct. 25 sp. 
Jurassic and Cretaceous ; Europe. MM. lata, Agass. (evii, 45). 
ARCOMYA, Agassiz, 1842. Shell very thin, much elongated 
transversely, sometimes compressed, sometimes more or less 
