CHAP. X. HYRIX.E. 283 



and its geographic distribution. All the species yet 

 known, are from the rivers of Tropical America and 

 Australia, — not one having been found north of the 

 line. Indeed, it may be almost said that the range of 

 the genus Unio is bounded by the same latitudes ; for 

 of all the UxioxiN^ we have yet seen, not more than 

 two have been brought from South America, and these 

 do not belong to the most typical groups. The Hv- 

 RiN'iE are distinguished by the angulated or wedge- 

 shaped form of the shell, and the winglike projection 

 of its hinge or dorsal extremities. The second cha- 

 racter is in the narrow linear form of the cardinal 

 teeth, which are on one side of the bosses, not beneath 

 them : these teeth also diverge to the anterior edge, 

 and are always more or less linear. The third cha- 

 racter is so very simple and constant, that it may be 

 recognised by the merest tyro ; the bosses or summits 

 of the valves are wrinkled or grooved transversely ; that 

 is, in a line from the apex to the basal margin. This 

 singular peculiarity, which pervades nearly all the types 

 we have yet seen of the Hyrin^, is directly opposed to 

 the characters of the Uxionin^, where, if the summits 

 are wrinkled, the wrinkles are placed lonyitudinaUy, or 

 in a direction from the anterior to the posterior ex- 

 tremities. In what way this may be connected with 

 the natural habits of the animals, is totally unknown. 



(265.) The passage from the Unionin.e to the Hy- 

 RiNiE is through Mysca in the former, and Iridea in 

 the latter. The gradation from one to the other is 

 beautifully perfect. Iridea has the strongest possible 

 affinity to Mysca, in the general shape of the shell, and 

 in the structure of the cardinal teeth : the form, how- 

 ever, is more oval than elongated ; the ends more 

 obtuse ; the posterior end of the hinge margin more 

 elevated, and the summits smaller. These, however, 

 are points of inferior moment. The chief distinction 

 is in the length and narrowness of the outer cardinal 

 tooth, and the almost total absence of the other : hence 

 it is, that in such shells there are always two distinct 



