331 



DONACID^. 



This small tribe is nearly allied to the last. The shells 

 are usually of much stronger and more compact texture, 

 and their hinges, though variable, more highly developed, 

 and provided with conspicuous primary teeth. The animal 

 is often more or less brightly coloured, and is strikingly dis- 

 tinguished by the great development of the cirrhi around 

 the orifices of the rather strong siphons, those of the bran- 

 chial tubes being more or less pinnated and ciliated. The 

 margin of the mantle, which was almost always fringed in 

 the last tribe, in this is indifferently plain or cirrhige- 

 rous, even in the same generic group. The foot is very 

 large, thick, sharp-edged, and not furnished with a byssal 

 groove. The species of this tribe live buried in sand, 

 most of them near the water's edge, and are, on the whole, 

 members of southern and even tropical regions. They 

 rarely occur in the fossil state, their littoral habits being- 

 unfavourable to their preservation. 



DONAX, LiNN^us. 



Shell rather strong, more or less triangularly wedge- 

 shaped, equivalve, very inequilateral ; posterior side short- 

 est ; surface smooth, or radiato-striate, or decussate, 

 covered by a distinct epidermis. Inner margin plain or 

 denticulated ; muscular impressions rounded or oblong ; 

 pallial sinus wide and deep, its outer edge rather distant 



