496 MANUAL OF THE MOLLUSCA 
Type, T. pubescens, Pl. XXIII., Fig. 9. 
Shell oblong, nearly equivalve, sightly compressed, attenuated 
and gaping posteriorly, smooth, or minutely scabrous; cartilage 
processes thick, not prominent, with a crescentic ossicle ; pallial 
sinus shallow. Outer shell layer composed of distinct, nucleated 
cells. 
Animal with the mantle closed; foot lnguiform; siphons 
rather long, separate, with fringed orifices; gills single, thick, 
plaited; palpi narrow, pointed. 
T. concentrica and J’. distorta, Mont., are found in the crevices 
of rocks, and burrows of Saxicuva ; they have been mistaken 
for boring-shells. 
Distribution, 17 species. Greenland, United States, Norway, 
Britain, Mediterranean, Canaries, China, Sooloo; 4—110 
fathoms. 
Fessil, 86 species. (Trias?) Lower Oolite—. United States, 
Europe. 
PHOLADOMYA, G. Sowerby. 
Recent Type, P. candida. Pl. XXII., Fig. 15. I. Tortola. 
Shell oblong, equivalve, veutricose, gaping behind; thin and 
translucent, ornamented with radiating ribs on the sides; lhga- 
ment external; hinge with one obscure tooth in each valve; 
pallial sinus large. 
Animal with a single gill on each side, thick, finery plaited, 
grooved along its free border, the outer lamina prolonged 
dorsally ; mantle with a fourth (ventral) orifice. (Owen.) 
Distribution, 1 species. ‘Tropical Africa. 
Fossil, 160 species. Lias—. United States, Europe, Algeria, - 
Thibet. 
Homomya (hortulana), Agassiz. Shell thick, concentrically 
furrowed, withe is «idiating ribs; 12 species. Oolites, Hurope. 
Tyleria, Adams. Cartilage inserted in a spoon-shaped hollow ; 
interior of shell with a layer of carbonate of lime between the 
spoon-shaped hollow and the anterior edge. 
MyactrEs (Schlotheim), Bronn. 
Synonyms, Myopsis (Jurassi), Agassiz. Pleuromya, Agassiz. 
Arcomya (Helvetica), Agassiz. Mactromya (mactroides), Ag. 
Anoplomya (lutraria), Krauss. 
Example, M. sulcatus, Fleming. (Allorisma, King, Pal. Tr., 
1850, Pl. XX., Fig. 5.) 
Shell oblong, ventricose, gaping, thin, often concentrically 
