Mr. Barnes on the Genera Unio and Alasmodonia. 127 



gitudinally, in this transversely, in that they are crowded and 

 small: in this they are distant and rather large. The ele- 

 vated ridge in that is higher and narrower; in this it is 

 broader and more depressed ; in that it continues of nearly 

 the same breadth to the base ; in this it diverges at the base, 

 to about four times its breadth at the beaks. The shell 

 above described has the appearance of age. The tubercles, 

 as well as the beaks are much corroded, and the epidermis 

 is cracked and broken in many places. 



Remarks on the first section, viz. 

 * Cardinal teeth, very thick. 



To this section belong the U. Peruviana, ligamentina and 

 ohliqua of M. Lamarck, and the U. Cylindricus? of Mr. 

 Say. The shells in this section bear in many respects, a 

 resemblance to each other. They are all thick, and have 

 a very strong hinge, with, in most cases, deeply sulcated 

 cardinal teeth, and a cavity under the beaks, more or less 

 angular and compressed, extending under the cardinal tooth. 

 They are nearly all waved, wrinkled, or tuberculated on the 

 outside. From the last two characters, however, some 

 varieties of the U. Crassus are excepted, which have little 

 or no cavity under the beak, and a small external surface. 



It may perhaps be thought that we have made too many 

 distinctions in this section, and that several of the fore- 

 going ought to belong to the U. Crassus, but they are 

 much more unlike than many which are admitted to 

 be distinct species, and therefore they require a sepa- 

 rate description. And when it is observed that we have 

 -not yet enumerated all that have been supposed to be- 

 long to the numerous family of the Crassus — that the ascer- 

 tained varieties of that species have already been described 

 to the number of eleven from (a) to (/) inclusive ; and that 

 among these varieties are several which M. Lamarck has de- 

 scribed as different species — and that the foregoing are all 

 very distinct from each other, so as to be instantly recogniz- 

 ed by even an inexperienced observer — we shall perhaps be 

 justified in discriminating the above, and several others also, 

 which belong to the next section. 



(To be conlinuid.) 



