496 MANUAL OF THE MOLLTJSCA 



Type, T. pubescens, PL XXIII., Fig. 9. 



Shell ohlon^, nearly equivalve, slightly 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 linguiform ; siphons 

 rather long, separate, with fringed orifices ; gills single, thick, 

 plaited ; palpi narrow, pointed. 



T. concentrica and T. distorta, Mont. , are found in the crevices 

 of rocks, and bui'rows of Saxicava ; they have been mistaken 

 for boring- shells. 



Distribution, 17 species. Greenland, United States, Norway, 

 Britain, Mediterranean, Canaries, China, Sooloo ; 4 — 110 

 fathoms. 



Fossil, 36 species. (Trias ?) Lower Oolite — . United States, 

 Eui'ope. 



Pholadomya, G. Sowerby. 



Becent Type, P. Candida. PI. XXII., Fig. 15. I. Tortola. 



Shell oblong, equivalve, ventricose, gaping behind ; thin and 

 translucent, ornamented with radiating ribs on the sides ; liga- 

 ment external ; hinge with one obscure tooth in each valve ; 

 pallial sinus large. 



Animal with a single gill on each side, thick, finely 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, witho iii i idiating ribs; 12 species. Oolites, Europe. 



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. 



Myacites (Schlotheim), Bronn. 



Synonyms, Myopsis (Jurassi), Agassiz. Pleuromya, Agassiz, 

 Arcomya (Helvetica), Agassiz. Mactromya (mactroides), Ag. 

 Anoplomya (lutraria), Ejrauss. 



Example, M. sulcatus, Fleming. (Allorisma, King, Pal. Tr., 

 1850, PI. XX., Fig. 5.) 



Shell oblong, ventricose, gaping, thin, often concentrically 



