ANATINID^, 141 



fossil Corim3^fe. But among the enormous number of known 

 Pholadom^'oe, we meet similar variations in one and the same 

 genus. 



Mytilimeria, Conrad, 1831. 



])istr.—2 sp. 31. Nuttalli, Conr. (cviii, 68). 



Shell rounded-oval, more or less ventrlcose, equivalve, fragile, 

 covered by a thin caducous epidermis ; beaks subspiral ; hinge 

 without teeth, but formed of small linear excavations under the 

 beaks to receive the ligament, which contains a small ossicle ; 

 muscular impressions small ; pallial impression with an obtuse 

 sinus. 



Animal gregarious, forming a nest. 



Edmondia, Koninck, 1842. 



Distr. — Fossil, 4 sp. Carb. — Permian ; Europe. E. unioni- 

 formis, Phil, (cvii, 39). 



Shell oblong, equivalve, thin, concentrically striated, close; 

 umbones anterior ; ligamental grooves narrow, external ; hinge- 

 line thin, edentulous, furnished with large oblique cartilage- 

 plates, placed beneath the umbones, and leaving space for an 

 ossicle ? or the plate may be equivalent to the subumbonal blade 

 in Pholas ; pallial line simple ? 



Cardiomorpha, Koninck, 1842. 



Distr. — Fossil, 38 sp. Lower Silurian — Carb. ; N. America, 

 Europe. G. excemtrica, Agass. (cvii, 40). 



Type, G. ohlonga (Isocardia), Sowerby (not Koninck). Car- 

 boniferous. 



Shell Isocardia-shaped, smooth or concentricallj^ furrowed, 

 umbones prominent, hinge edentulous ; hinge-margin with a 

 narrow ligamental furrow, and an obscure internal cartilage- 

 groove. 



Ceromya, Agassiz, 1842. 



Etym. — Keraos, horned ; mya, mussel. 



Distr. — Fossil, 14 sp. Inferior Oolite, Greensand ; Europe. 

 G. Aalensis, Quenst. (cvii, 41, 42). 



Shell Isocardia-shaped, slightly inequivalve ? very thin, gran- 

 ulated, often excentrically furrowed ; ligament external; hinge 

 edentulous ; right valve with an internal lamina behind the 

 umbo ; pallial line scarcely sinuated ? 



The Ceromyse are principally characterized by their tumid, thin 

 and concentrically laminated shell, distant beaks, and the oblique 

 furrows which are externally traceable. Chenu unites this to 

 Cardiomorpha, but in that genus there does not appear to be 

 any such arrangement for the attachment of an internal liga- 

 ment (similar to that of Lyonsia) ; there is along the raised 



