288 



SHELLS AND SHELL-FISH. 



PART I. 



again^ has the rudiments of a very thin and short lateral 

 plate. It may be proper to observe in this place, that 

 Dipsas was the original name imposed by Dr. Leach to 

 this sub-genus ; but Larentini, so far back as 1768, had 

 given this name to a genus of reptiles : and as this is 

 an insuperable bar to its repetition, we gladly adopt that 

 of Symphynota Lea, under the restrictions here men- 

 tioned. 



(270.) The last sub-family is that of the Alasmo- 

 DONTiN^, the typical species of which are all from the 

 rivers of America. They are connected to the anodons, 

 in the most graduated and perfect manner, by our genus 

 Hemidon; and the first type is probably represented by 

 the Unio calceola of Lea : then follows the typical form, 



as seen in Say's original 

 Alasmodon undulata 

 (Jig. 61.); our genus 

 Uniopsis appears to suc- 

 ceed this , then we have 

 the elongated form in 

 Schumacher's Margari- 

 tana ; and, finally, the 

 anodoniform, or most 

 aberrant type, is seen in 

 our Complanaria gigas and rugosa.* The whole of these 

 constitute one of the most natural divisions in the family. 

 Typically considered, they are without any distinct lateral 

 teeth ; the hinge-plate being either long and merely con- 

 vex, as m. Mm-garitana ; or short, flattened, and obliquely 

 grooved, as in Complanaria. The cardinal teeth are 

 always present, but vary in each type ; hence they fur- 

 nish an admirable clue to the analogies of the genera. 

 Nothing appears known of the animal; but this is not 

 material to our present purpose, since the variation of 

 the shell is quite sufficient to guide us in their natural 



• Mr. Lea, in liis artificial arrangement, places these two shells in widely 

 different divisions, because one, he says, is " symphynote," and| the other 

 " nort-symphynote: " they are the Alas77iodo>ita compla7iata and rugosa of 

 American writers, and are most naturally connected. 



