BIVALVES. DONAX. 23 



DONAX. — Wedge-shell 



Animal — a Tethys: Shell hivalve, with generally a crenu- 



late margin, the frontal margin is very obtuse ; hinge :..>■ */^^ ^^'^i 

 with two teeth, and a single marginal one placed a little 

 behind, rarely doable or triple. 



The most leading characteristic of the Donax is derived 

 from its form, which (throughout the nineteen species) is 

 similar to that of a wedge, being very broad and thick at 

 one extremity, and gradually narrowing and lessening to 

 the other. The frontal margin is generally very obtuse, 

 and the anterior slope is not unfrequently furnished with 

 a sort of fissure or gape; near to which is situated a carti- 

 laginous ligature or ligament, which prevents the two 

 shells from separating when the animal has occasion to 

 open them. 



Some of this genus, however, are a little ambiguous 

 in their external appearance ; so much so, that they fre- 

 quently hold forth an inducement to rank them among 

 the species of the Venus ; but in these cases the hinge 

 alone must be the guide, which in the Donax is furnished 

 with two teeth, and a single marginal one placed some- 

 what behind, not often doubled or tripled. 



The exterior of the Donax is mostly of a smoothish sur- 

 face, though many are covered with nearly obsolete lon- 

 gitudinal striae, being embellished at the same time with 

 numerous reddish or purple rays, diverging from the beaks 

 to the margin. Other species are perfectly rugose or 

 rough on their outside, which is caused by crowded strise 

 crossing each other in a longitudinal and transverse di- 

 rection; this disposition of the striee gives the shell a fo- 



