264 



SHELLS AND SHELL-FISH. 



PART I. 



(249.) The primary divisions of this family^ as we 

 stated some years ago, are typically represented by the 

 following genera: — Unio, Hyria, Iridina, Anodon of 

 Lamarck, and Alasmodon of Say.* Having shown that 

 these form a circular group, it follows, that these divi- 

 sions are sub-families, to which we give the usual ter- 

 mination in ina. Their secondary groups are conse- 

 quently genera, and their third are sub-genera. We shall 

 now take a rapid survey of each in their natural order 

 of succession. 



(250.) The Unionin^ stand at the head of the 

 family ; and their typical characters may be thus briefly 

 stated : — The form is various, but the two cardinal 

 teeth t and the anterior lateral teeth are always present; 

 the umbones, or bosses, are sometimes wrinkled longitu- 

 dinally, but never sulcated transversely, — this latter 

 character being peculiar to Hyria. Thus detached by 

 the most simple and definite characters furnished by 

 the shell, we find the whole of the species will arrange 

 themselves under one or other of the following genera : 

 — In Unio pre-eminently so called, the umbones, or 

 bosses, are very large, thick, and prominent; and the car- 

 dinal teeth (^jig. 4?. «) 

 either in an obliquely 

 perpendicular line with 

 the base of the shell, or 

 receding in a contrary 

 direction, to the ante- 

 rior margin. The 

 form is various ; but 

 the hinge margin, al- 



* Mr. Lea, in his valuable paper on Unionidce (Am. Tr.), observes that I 

 have stated" difficulties in adopting these genera." I am not conscious of 

 ever having done so, and have taken every occasion of expressing my be- 

 lief that they are perfectly natural. As I entirely dissent from the views 

 of this gentleman, both in his divisions, in his genera, and even in many of 

 his species, I am anxious not to be misunderstood, as having in any way 

 changed my opinion on this family. Mr. Lea has, however, added many 

 interesting and beautiful species, for which hedeserves the thanks of everj 

 naturalist ; we only regret that all his arrangements of them aj>pear to us so 

 very artificial and perplexing. 



f I consider these teeth as two ; not as one, cl^t into two. 



