CHAP. X. SUBGENEBA OF UNIO. 269 



our Megadomus fills up the interval between such spe- 

 cies as Lamarck's Peruvianus and JEsopus ; but this 

 would exclude the European Unto sinuata, littoralis, 

 &c., which seem closely united to the typical or Ame- 

 rican group by their short, oval shape, and the thickness 

 of their cardinal teeth. On the other hand, we feel 



persuaded that our Po- 

 tomidce must be kept 

 distinct from the sub- 

 genus Unio; and this 

 will be apparent to every 



^SH^gHIiUI one wno looks to tne 



^ 1] ffl annexed cut of the car- 

 i dinal teeth of Unio My- 

 / tilo'ides (fig. 52.),, and 

 those of Potomida corrugata (fig 51.). 



(253.) Two of these sub-genera, being more than 

 usually perfect in their types of form, or, in other 

 words, in the diversity of their species, will next be 

 glanced at, for the purpose of showing there is but one 

 principle of variation throughout the entire family. 

 We shall first take the sub-genus Unio as here re- 

 stricted. The types of this are seen in those inequi- 

 lateral shells which have been sent to us from America, 

 as the U. Mytilo'ides Raf. and the undatus of Barnes. 

 Next follow those which are nearly round and equilateral, 

 such as U. Torsus Raf. Our U. truncatus, very much 

 elongated, is the third type of form. The fourth, which 

 represents Theliderma, is perhaps the cornutus of Barnes ; 

 while the fifth, which unites the last to the first type, 

 is an American species, formed like obliqua Lea, but 

 with small scattered tubercles down the middle. * This 

 unites with Msopus, and again brings us back to the 

 undatus. Thus, in the confines of a single sub-genus, 

 are types of JEGLIA, in U. Torsus ; IRIDINA, in U. trun- 

 catus; ANODON, (through THELIDEBMA), in U. nodosa ; 



* I pretend not to enter upon specific names, for nearly all those which 

 belong to species unfigured appear to me one mass of confusion. 



