INVERTEBRATE PALEONTOLOGY. 229 



angles from close up under l lie hinge-line, beneath the beak of I lie righl valve 

 (the posterior tooth being larger and compressed, and the anterior pointed), 

 and one prominent and one rudimentary cardinal tooth under thai of the lefl ; 



lateral teeth in : ligamenl external; fulcra short and erect. (Pallial line 



unknown.) 



The foregoing diagnosis is intended to include three sections that seem 

 to fall within one generic group, although at one time I regarded two of them, 

 then only known from mere external characters, as being rather widely sepa- 

 rated from each other. These subordinate sections may be distinguished as 

 stated below : 



1. liopistha, Meek (typical). 



Shell transversely subovate, ornamented, excepting on the poste- 

 rior dorsal portions of the valves, by regular, simple, well-defined, 

 sometimes subcrenate, radiating costse. — "(Type as already stated.) 



2. cymella, Meek. 



Shell of much the same form as the last, but differing in having 

 only large, strong, regular, concentric surface-undulations, crossed by 

 a few impressed, radiating, linear markings on the middle portions 

 of the valves. — Pholadomya in/data, M. & H. 



3. psilomya, Meek. 



a. Shell agreeing with the above sections in form and thinness, 

 or sometimes a little more- gibbous, but having the radiating stria 1 or 

 ridges nearly or quite obsolete, and the radiating rows of granules, 

 or spine-base, usually more distinct. — Poromya lata, Forbes. 



b. Shell large, thick, with well-defined concentric furrows and 

 ridges. — Poromya superba, Stoliczka. 



This genus is undoubtedly allied, in some of its characters, to Poromya, 

 Forbes, as may be inferred from the fact that two of the East-Indian Creta- 

 ceous species believed to belong to it, were actually referred by Professor 

 Forbes, the founder of the genus Poromya, to that group. This faci would 

 deter me from referring these Cretaceous shells to any other genus, were 

 it not that they certainly disagree in their hinge-characters (as well as in 

 general form) with the detailed description of the hinge and other pecul- 

 iarities of the recent typical species of Poromya, published by Forbes and 



