CHAP. X. 



SUB-FAMILY OP UNIONIxXJE. 



265 



though sometimes angulated, in the most aberrant groups, 

 is never connate ; nor is the shape of the shell properly 

 cuneate. In the next genus, Mglia Sw. {h), the car- 

 dinal teeth are lengthened, compressed, and take a di- 

 rection precisely opposite to the above ; they diverge, 

 in fact, from the apex towards the anterior margin 

 instead of from it : the inner tooth is smallest, and not, 

 as in Unio, the largest. The shell is generally cuneate 

 and the bosses prominent. None of the types are con- 

 nate or tuberculated. In Mysca the shells are more or 

 less transversely elongated, much thinner, and the bosses 

 much smaller than in the two preceding genera; and 

 the cardinal teeth differ essentially from both : they are 

 much compressed, irregularly crenated, and placed as 

 nearly as possible on a parallel line with the anterior 

 margin; they are also sometimes so thin and slightly 



divided, that they ap- 

 pear almost blended into 

 one : hence they are 

 rarely obtuse, nor is the 

 shell winged or tubercu- 

 lated. In Lymnadia alata 

 {jig. 48.), the posterior 

 part of the hinge margin 

 is greatly elevated, and 

 the shell is strictly con- 

 nate ; while in Megado- 

 mus, of which our M. 

 gigas is the type, the cardinal teeth are like those of 

 Unto, but there is only one lateral tooth in each valve. 



(251.) To these gTOups we shall presently return ; 

 in the mean time it will be as well in this place to show 

 how perfectly they represent the five primary divisions 

 of the whole family. It is immaterial to our present 

 object, whether the genus Mysca contains a portion of 

 those shells we have arranged under the name of Poto- 

 mida, since none of these latter can be looked upon as 

 typical examples of Mysca. 



