142' ANATINIDiE. 



Shell oval, transverse, depressed, closed, inequivalve, subin- 

 equilateral ; right valve the larger, with one pja-amidal tooth, 

 and a narrow and deep socket ; left valve with two unequal teeth 

 separated by a large socket. Ligament internal, pallial impres- 

 sions simple, slightly inflected posteriorly. 



Animal with the mantle united behind, margins of the mantle 

 •with duplicate foliaceous tentacles ; foot compressed, triangular; 

 siphons short, united at the base, the incurrent tube the larger 

 and more elongated, the opening of which is surrounded by 

 arborescent tentacles. 



DORSOMYA, Ryckholt, 1852. The shell resembles Corbulomya 

 in shape, but the knowledge of the hinge is necessary for its 

 correct generic determination. Carboniferous ; Belgium. B. 

 dorsata. 



Cryptomya, Conrad, 1848. 



Distr. — 8 sp. California, Australia, Philippines. G. Philip- 

 pinarum, A. Ad. (cvi, 21). 



Shell inequilateral, transverse, oblong, gaping behind ; valves 

 with radiating, sometimes crossed by concentric striae; right 

 valve with a lamellar tooth, left valve with a broad fosset ; liga- 

 ment internal ; pallial impression with a small sinus. 



Siphons short; not covei-ed as in Mya with a coriaceous epi- 

 dermis. 



Family ANATINID^. 



Shell often inequivalve, thin ; interior nacreous ; surface 

 granular; ligament external, thin ; cartilage internal, placed in 

 corresponding pits and usually furnished with a free ossicle ; 

 muscular impressions faint, the anterior elongated ; pallial line 

 usually sinuated. 



Animal with mantle-margins united ; siphons long, more or 

 less united, fringed ; gills mostl}^ single on each side, the outer 

 lamina prolonged dorsally beyond the line of attachment. 



Pholadomya and its fossil allies have an external ligament 

 only ; no ossicle. The external surface of these shells is often 

 rough with large calcareous cells, sometimes ranged in lines, and 

 covered by the epidermis ; the outer layer consists of polygonal 

 cells, more or less sharply defined ; the inner layer is nacreous. 



The most impoi-tant distinctions of the shells are their thin 

 pearly structure and usually edentulous hinge. These at least 

 are almost the only characters upon which we can depend in the 

 determination of fossil species. The shells are generally inequi- 

 lateral, but sometimes the anterior, sometimes the posterior side 

 is the longer, and the latter usually has a wide gap at the end. 



There is a large amount of variation in the form of the 

 animals, but they all have the mantle-margins united, with an 

 opening at the antero-inferior side for the protrusion of a small 



