142 



Genus 6. GNATHODON, Gray. 



Animal; with the mantle freely open in front ; margins plain; 

 siphons short, partly united ; foot very thick, tongue-shaped. 



Shell ; equivalve, obliquely oval, rather gibbous, covered with an 

 olive-green epidermis; hinge composed of a central and two 

 lateral teeth in one valve interlocking with two central and two 

 lateral teeth in the other, enclosing the ligament in a deep cen- 

 tral pit. 



This genus was instituted for the reception of a bivalve of quite peculiar 

 character, inhabiting Lake Pont-Chartrain, New Orleans. The animal has 

 the mantle freely open in front for the passage of a large tongue-shaped 

 foot like that of Anoclon and Unio, and the shell has the same freshwater 

 aspect, being covered by an olive-green epidermis, more or less eroded to- 

 wards the umboes. The shell internally is, however, of a more transparent- 

 white substance, and the hinge has its ligament enclosed, like Mactra and 

 Crassatella, within an internal cavity. Only one species is known. 



Figure. 



Gnathodon cuneatus. PI. 40. Pig. 218. Shell, with one valve dropped 

 to show the strong dentition of the hinge on either side of a central 

 pit in which the ligament is contained.* 



Genus 7. MACTRA, Linnceus. 



Animal ; triangular or oblong, its mantle freely open in front as 

 far as the siphons, the margins more or less distinctly fringed ; 

 siphons united to their extremities, which are surrounded with 

 fringes of simple cirrhi ; foot strong, changeable in shape, lin- 

 guiform and geniculated. fForbes.J 



Shell ; ovately triangular or oblong, a little gaping at the sides ; 



* " Gnathodon cuneatus was formerly eaten by the Indians. At Mobile, on the Gulf of 

 Mexico, it is found in colonies along with Cyrena Carolinensis burying two inches deep in banks 

 of mud ; the water is only blackish, though there is a tide of three feet. Banks of dead shells 

 three or four feet thick are found twenty "miles inland. Mobile is built on one of these shell- 

 banks. The road from New Orleans to Pont-Chartrain, six miles, is made of Gnathodon shells 

 procured from the east end of the lake, where there is a mound of them a mile long, fifteen feet 

 high, and twenty to sixty yards wide ; in some places it is twenty feet above the level of the 

 lake." — Lyell. 



