DONAX WEDGE SHELL. 163 



thick, and obtuse at the anterior end, and gra- 

 dually narrowing and lessening at the posterior ; 

 the margin is often crenulate, and usually gaping ; 

 the hinge has two central teeth in each valve, 

 and one remote lateral one ; the ligament is ex 

 ternal. 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



The wedge-like form of the Donaces easily 

 distinguishes them. The derivation of the name 

 from toval (donax) an arrow, may have been 

 adopted either as indicative of the sagittate form, 

 or of the rapidity with which, in consequence of 

 its peculiar shape, the animal can dart into the 

 sand, whenever danger threatens. One species 

 (Donax Irus) perforates the hardest limestone, 

 but by what means it effects such a lodgment has 

 not been ascertained ; pieces of stone are often 

 thrown ashore on the Devonshire coast filled 

 with these shells fitting into the apertures which 

 they have pierced. The shells of this genus, 

 are generally of a fine rich purple colour, some- 

 times marked with rays of purple on a white 

 ground. Many of the species are of a yellow 

 olive hue. The animal has two long slender 

 tubes not joined together, and a lamellar foot. 



