148 ANATINID^. 



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. (cvii, 44). 



Shell transversely elongated, very thin, elliptical, equivalve, 

 ver}'- 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. 



Myacites (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. 



HOMOMYA, 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 

 distinct. D'Orbigny has found cardinal teeth in well-preserved 

 specimens, and refers Pleuromya to Panopeea. Several sp. 

 Liassic and Jurassic P. tenuii<triata , Agass. 



MYOPSis, Agassiz, 184,5. Shell moderate or large sized, covered 

 with linear points forming radiating lines, moi'e or less gaping 

 at both extremities ; a cardinal tooth in each valve ; beaks more 

 or less anterior, sometimes even marginal ; pallial impression 

 with a profound sinus ; muscular impressions indistinct. 25 sp. 

 Jurassic and Cretaceous; Europe. M. lata, Agass. (cvii, 45). 



ARCOMYA, Agassiz, 1842. Shell very thin, much elongated 

 transversely, sometimes compressed, sometimes more or less 



