61 



CHAPTER XI. 



OF THE MACTRA. GENUS VIII. 



MACTRA. 



"Animal a Tethys. Shell bivalve, with unequal sides, and equivalved ; 

 hinge, the middle tooth folded, with a small hollow adjoining ; lateral 

 teeth remote and inserted." — Linn. 



Phis genus is easily known by the hollow (jA. 2. f. 20. a) un- 

 der the beak, between the teeth, which is found in both valves, 

 and serves to hold the cartilage, which is thus wholly internal. 

 Tiiere is frequently on one side of the hollow a diverg-ing 

 tooth like two sides of a triangle (p/. 2./. 20. Z>) in eitlier one or 

 both valves. Some species, as M. solida, stn'atula, stidtorum, 

 &c., have lateral teeth, but a few are without, as 31. lutraria. 



The cartilage internal, and the hollow for it being accom- 

 panied by teeth may be considered as the essential character 

 of this genus, by which it may clearly be distinguished from 

 eveiy other ; it approaches most to the Mya ; but in that the 

 hollow for the cartilage has no teeth accompanying it, and 

 stands erect. 



The animal is closely allied to the Cockle ; it has two tubes, 

 which project but a short distance beyond the edge of the 

 shell : it also protrudes a foot at the front, towards the lower 

 end, like the Cockle. The lines radiating from the beak to 

 the margin are called longitudinal ; and those that are parallel 

 to the margin are called transverse. 



