142 ANATINIDA. 
Shell oval, transverse, depressed, closed, inequivalve, subin- 
equilateral ; right valve the larger, with one pyramidal tooth, 
and an: Wwrow 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. 
porsomyA, Ryckholt, 1852. The shell resembles Corbulomya 
in shape, but the knowledge of the hinge is necessary for its: 
correct generic determination. Carboniferous; Belgium. D. 
dorsaia. 
Crypromya, Conrad, 1848. 
Distr.—8 sp. California, Australia, Philippines. C. Philip- 
pinarum, A. Ad. (evi, 27). 
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 covered as in Mya with a coriaceous epi- 
dermis. 
Famity 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 mostly 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 important 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 i 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 
