CHAP. X. 



UNIONIN^. 



275 



MyscAj which they nevertheless represent by their pod- 

 like form. The remarkable T. cylindrica, already noticed^ 

 unites to this form the tubercles of the sub-genus The- 

 LiDERMA, which has just been analysed. 



(258.) There seems strong evidence for believing that 

 our Unio gigas follows Theliderma ; in which case it 

 may be regarded as the type of a distinct sub-genus, 

 which, for the present, we shall name Megadomus. It 

 differs from the T. Peruviana and its allies, by having 

 only one lateral tooth in each valve ; and the bosses are 

 remarkably small and compressed. We have already 

 alluded to the disappearance of one of the cardinal teeth 

 in an aberrant species of Theliderma ; and we can now 

 notice a second gradation in an enormous large and very 

 old shell, intimately resembling plica ta, but having the 

 second lateral tooth in one valve reduced to a mere 

 vestige. The question is, whether this deficiency is 

 accidental or permanent : all such deviations from the 

 typical characters of the Unionin^, particularly in their 

 teeth, we regard as prototypes of the ALASMODONTiNiE ; 

 but their natural location is very difficult, particularly 

 until the question regarding Potomidea being a genus, 

 or a sub-genus, is decided. 



(259.) ^GLiA, the second genus, is the represent- 

 ative of Hyria : this is apparent in the general form of 

 their shells, and in the structure of the cardinal teeth. 

 The posterior half is very thick and truncated, so as to 

 form a carinated angle; the bosses approach towards 

 the centre, and are very prominent; the form is thus 

 almost equilateral, and consequently the lateral teeth are 

 T 2 



