80 ANODONTA. UNTO. 



'23. We distinguish those Mussels which have the hinge placed 

 lower, under the name of MODIOLUS, and another is called 

 LITHODOMUS, which hns the shell almost equally rounded at both 

 ends, and excavates holes in stones, to which it was at first only 

 suspended. 



Fig. 100. ANODONTA. 



24. The ANODONTJE live in fresh waters, and are recog- 

 nised by their thin shell, moderately inflated, oval, close, and 

 without teeth in the hinge (Jig. 100); they want the byssus, and 

 are provided with a very large foot, compressed and almost 

 quadrangular in form, by means of which they crawl on the 

 sand or mud. A great many species are found in the great rivers 

 of the United States. 



25. The UNIONES closely resemble the Anodonta), but have a 

 more complicated hinge, the right valve having a pit (fossette) 

 into which a tooth of the left valve penetrates, presenting behind 

 a long lamina, which, in its turn, is received between two 

 laminae of the opposite side. These mollusks also inhabit fresh 

 waters, but prefer those which are running. A very great num- 

 ber of species are found in our Western waters, for the descrip 

 uon of most of which we are indebted to the labours of Mr. I. 

 Lea, of Philadelphia. 



23. What is the genus Litho'domus ? (From the Greek, lithos, a stone, 

 and the Latin, dornus, a house.) 



24. How is the genus Anodo'nta recognised? (Anodonta, from the 

 f Jreek, </, without, and odous, in the genitive case, odontos, a tooth.) 



25. What are the characters of the genus U'nio ? (Unio, Latin, % 

 pearl.) 



