UNIONIN^. — THELODERMA. 



271 



totypes of all the natural divisions, large or small, of the 

 Unionjd.s:. 



(255.) The other sub-genus, which contains sec- 

 tional types, is Theliderma ; and this we shall now 

 briefly notice. It will be remembered that we have 

 placed this sub-genus immediately after Ligumia, the 

 type of which is the Unio recta of Lamarck. Nearly 

 all these shells have their posterior margin dilated 

 and angulated, and thus represent the genus Ano- 

 don of Lam. Although the species are remarkably va- 

 ried, they are all known by the external surface being 

 covered by numerous tubercles of different shapes, which 

 stud every part of the shell. Where we find a cha- 

 racter, however trivial it may appear, pervading a whole 

 group, we may be perfectly assured that it is a natural 

 character, although it may not be the only one. 



(256.) Now, let the conchologist place before him 

 the following shells (fig. 54^.); — 1. Irrorata (a); 2. 



Metanevra ih) ; S. Cylindrica (c) ; 4. Pustulata* 

 Sw. {d); and, 5. Plicata (e); — and he will then have 

 what we consider the sectional types of Theliderma. 

 It is obvious that the two first have the greatest resem- 

 blance to each other ; both are nearly circular, and tu- 

 berculated ; but irrorata is not winged, like metanevra. 

 Now, this difference, trivial as it seems, is accompanied 



* We merely use this name provisionally, until we know that by which 

 it has been distinguished in America. 



