INVERTEBRATE PALAEONTOLOGY. 233 



consistently done, I frankly confess thai a very careful study of specimens of 

 each, showing clearly their hinges as well as their external characters, leads me 

 (o the conclusion that they cannot be properly separated more widely than 

 as well-marked subgenera of the same group, and even this lias to be done 

 mainly on external ornamentation ; that is to say, the surface of Liopistha 

 is ornamented by usually well-defined, regular, simple, radiating costas on all 

 parts of the valves excepting the posterior dorsal, and sometimes a little of 

 the anterior dorsal regions; while in Cymella, the surface is ornamented by 

 large, regular, rounded, concentric undulations, that are crossed only on the 

 middle portions of the valves by a few impressed, radiating lines. Both have 

 the same general form, the same extremely thin shell, and granular surface; 

 the larger granules in each being arranged in radiating rows, and presenting 

 the appearance of having been minute spine-bases. On comparing the hinges 

 we also find that in Liopistha proper, there are the same characters in nearly 

 all respects that have already been described in detail ; the only observable 

 difference being that in Liopistha the posterior cardinal tooth is not quite so 

 broad, directed a little more obliquely forward, with a slight anterior slope 

 Its anterior tooth of the right valve, as well as that of the left, agree well 

 with the corresponding teeth of that of Cymella; and although I have seen 

 no specimen showing clearly the posterior cardinal and pits of this valve, 

 it is evident from those of the fight valve that they must correspond very 

 closely with what we see in Cymella. Indeed, I do not think the differences 

 mentioned in the hinge-char acters alone, greater than we might expect to 

 see in different species of the same subgenus. For these reasons, lam com- 

 pelled to adopt the conclusion that these types should not be more than 

 subgenericaUy separated, especially as the Indian Cretaceous species men- 

 tioned seem, partly at least, to bridge over, as it were, the differences in 

 t lie surface-ornamentation described; being to some extent intermediate in 

 these characters, though they appear to form a third section of the same 

 genus. 



As already stated, Professor Forbes referred the East-Indian Cretaceous 

 species mentioned to Poromya, without knowing their hinge-characters. 

 Dr. Stoliczka, having the advantage of the extensive collections obtained 

 by the Indian geological survey, succeeded in making out the hinges of these 

 Cretaceous species quite clearly ; and. although he follows Professor Forbes 

 in referring them to Poromya, it will be observed that they do not agree in 

 30 a 



