360 MACTRIDiE. 



umbones are remarkably prominent ; the beaks are acute 

 and directly inflected. The anterior extremity is romided, 

 but with a alight angulation a little below the middle ; 

 the posterior side, which is much the longer of the two, 

 is attenuated at its termination and acutely angulated 

 below ; the umbonal ridge, which bounds the hinder dor- 

 sal area, is tolerably evident, being distinguished by the 

 roughness of the epidermis upon it. The interior is of a 

 pure glossy white. The lateral teeth are moderately elong- 

 ated, strong, and vertically sulcated, the front one flattened 

 at its apex where the hinder one is rounded. 



The majority of specimens are not an inch long, nor 

 quite three quarters of an inch broad : the Irish example, 

 delineated in plate XXI, is of unusual magnitude. 



The animal is triangular, and thick ; the edges of its 

 mantle are not so distinctly fringed as in the other Bri- 

 tish species, but rather crenated or serrated. The siphons, 

 which are united to their extremities, vary in colour from 

 reddish to yellowish, or white. Their orifices are thickly 

 fringed at the margins by a double row of cirrhi, and the 

 anal one often projects its tubular valve. The sides of the 

 siphons are scabrous at intervals, and the back of the anal 

 tube is rough with a serrated keel. The foot is large, taper- 

 ing, and of a yellowish white colour. 



This shell is universally distributed, and extremely com- 

 mon in all sandy localities around our coast, being equally 

 a littoral and moderately deep-water species, inhabiting 

 sand, sandy gravel, and even occasionally sandy mud. A 

 few localities will serve to mark its range in depth. Low 

 water in sand, Swansea and adjacent bays (Jeffreys) ; 

 Frith of Forth (E. F.) ; Clyde (Smith); and Orkney 

 (Thomas) ; in seven fathoms, Weymouth ; twelve fathoms, 

 Anglesey ; and four to seven fathoms, Zetland (M'An- 



