BIVALVIA. 173 



Gen. Char. Shell thick, strong, and capable of being perfectly closed by the 

 animal ; generally more or less inequilateral, with a slightly trigonal form, equivalved, 

 often smooth, sometimes rigid, or deeply furrowed on the exterior ; hinge with two 

 diverging cardinal teeth in the left valve, and a trigonal space between them for the 

 reception of a prominent tooth of the same form in the right valve, often striated ; 

 generally a lateral tooth on the anterior side beneath the lunule, and sometimes an 

 elevated ridge or tooth inside the dorsal margin on the posterior side ; two deep 

 impressions are left upon the shell by the adductor muscles,* and the impression of 

 the mantle is without any inflexion ; ligament external. 



Animal of the form of the shell ; edges of the mantle plain ? disunited, except 

 at the posterior, where they form two siphonal openings, with simple orifices, and 

 not extending beyond the shell ; foot rather small, strong, and somewhat of an 

 angular shape. 



The greater number of the species of this Genus have the inner margin of the shell 

 covered with crenulations, and their presence or absence was for a long time considered 

 a character of sufiicient importance for specific distinction, it is now well known that 

 those appearances are not to be depended upon for such a purpose, as in all the species 

 of this Genus the immature, or, at least, the young, state of the shell has its margin 

 smooth or free from crenulations, and never, until it has attained to maturity, does it 

 assume that character ; and whenever a specimen has its margin crenulated, it may 

 then be considered to have arrived at its full growth. 



English authors have generally adopted the above name, while upon the Continent 

 Crassina appears to have been more frequently employed ; there is no doubt, however, 

 that priority belongs to Mr. Sowerby, and that Lamarck's name is entitled to rank 

 only as third in point of date. 



This genus has a considerable vertical range in the living state, but it is more often 

 found in waters of considerable depth, and the greater number of known recent species 

 are inhabitants of the colder regions of the North. 



1. ASTARTE TRIANGULARIS, Montuffue. Tab. XVII, fig. 10 « — d. 



Mactra triangularis. Mont. Test. Brit., p. 99, t. iii, fig. 5, 1803. 



— MiNDTissiMA. Id. Test. Brit. Suppl., p. 37, 1808. 

 GOODALLIA TRIANGULARIS. TuH. Brit. Biv., p. "7, t. vi, fig. 14, 1822. 



— MiNUTissiMA. Id. „ „ t. vi, fig. 15, 1822. 

 Mactroibea triangularis. Brown. lUust. Brit. Conch. Syst. Ind., 1827. 

 Mactrina triangularis. Id., pi. IG, fig. 25. 



— MINUTISSIMA. /(/., 2d edit., p. 108, pi. xlii, figs. 25, 2(5. 

 AsTABTE SUBTRIGONA. S. Wood. Catalogue, 1840. 



* There is also in most of the species a deeply indented mark near the anterior adductor. 



