266 



SHELLS AND SHELL-FISH. 



PART I. 



Analogies of the Genera q/" UNiONiNi«. 



Genera of the 

 Unionince. 



Unto. 

 Mglia. 



My sea. 



Lymnadia. 

 Megadomus. 



Analogical Characters. 



Cardinal teeth thick, short 

 f Cardinal teeth compressed, ) 

 I lengthened; shell cuneate. J 

 r Shell lengthened; cardinal teeth') 

 ■J crenated, almost on a linewith > 

 t the lateral teeth. " j 



f Shell thin, winged; umbones") 

 (_ small. 3 



C Lateral teeth imperfect, one or 

 t none. 



Types of the 



Sub-families of die 



Unionida. 



Unio. 

 Hyria. 



] 



Iridina. 



Anodon. 

 Alasmodox. 



These analogies are very remarkable, particularly 

 when we confine our attention to the types of each of the 

 groups thus brought into comparison. Mglia ovata Sw. 



(Unio ovatus of Say) 

 (fig.4fQ.), like Hyria, has 

 the cardinal teeth as- 

 suming the shape and 

 the position of lateral 

 teeth ; and both have 

 merely two or three tu- 

 bercles immediately be- 

 neath the bosses. The 

 elongated form of all the species of Mysca reminds us 

 of Iridina, no less than the crenated edge and mar- 

 ginal direction of the cardinal teeth, li Lymnadia, again, 

 had no cardinal teeth, it would be a Dipsus of Leach ; 

 and both these have the very aspect of Anodon. Mega- 

 domus is the only type in which the lateral teeth are 

 imperfect ; hence a truer prototype of Alasmodon, 

 where both those teeth are entirely wanting, could 

 scarcely be found ; particularly as the cardinal teeth 

 are precisely like those of the typical genus Unio. Our 

 limits, and, indeed, our materials of specific nomen- 

 clature *, will not permit us to analyse the whole of 



♦ The specific descriptions of Lamarck are scarcely less unsatisfactory 

 than many of those recently published in America ; I have seen totally 



