248 



NATURAL HIS TOUT. 



ligament is external and short, the cartilage internal and long, the siphonal fold deep. Sixty 

 species are known from Brazil, India, China, &c. 



Genus Mesodesma-. * The valves of shell are thick, triangular, closed ; the ligament is internal ; 



there are lateral teeth in each valve ; the siphonal fold is- 

 small ; the muscular impressions are deep. Thirty- one species. 

 occur in the West Indies, Chili, and the Mediterranean. 



Genus Ervillia, "Lentil-shell." The shell is oval and small, 

 with a single prominent tooth to the hinge in right valve, 

 DONAX TKUNCTILUS. an( J two obscure teeth in left valve. The siphonal fold is deep. 



Two species are living at fifty fathoms in the West Indies, &c. 



In the genus Donax the shell is wedge-like, somewhat triangular, rounded in front, truncated 

 behind ; the border of the valves is crenulated ; there are two hinge teeth in each valve, and the 

 ligament is external. Sixty-eight species are known, found living in Norway, the Baltic, &c. 



The genus Galatea has a very thick, wedge-shaped, triangular shell, with an olive-green epidermis ; 

 the hinge is strong, with three teeth and an external prominent ligament, the siphonal fold is 

 distinct. This is a fresh- water shell, inhabiting the rivers of Africa. Six species are known. 



SOLEN EXSIS. 



FAMILY XIX. 

 The shell is more or less elongated, open at each end ; the hinge ligament is external ; there 



are cardinal teeth, two in the right valve and three in the left. The Solem have a large and 



powerful cylindrical foot ; the siphons are short, and the gills narrow. 



Genus Solen, "Razor-fish." The shell is somewhat cylindrical, long and straight, or slightly 



curved, margins parallel, ends 



gaping, hinge line elongated, 



ligament external, foot cylin- 



drical, obtuse. The Solens are 



of world -wide distribution, 



except in the colder seas. The 



Razor- fishes are powerful bur- 



rowers ; they never willingly leave their burrows ; they may, however, be caught with a bent wire, 



and are good eating when cooked. 



Genus Solecurtus. The shell is ovate oblong, the umbo small, margins almost parallel, ends rounded^ 



gaping ; the hinge ligament is external ; there are two hinge teeth in each valve ; the siphonal fold is 



very deep ; the animal is 

 entirely retractile within 

 the shell. The Solecurti 

 bury themselves in sand 

 and mud, and are difficult 

 to obtain alive. S. cari- 



bj',us occurs in countless numbers in the bars of American rivers. By removing three or four 



inches of sand its burrows may be discovered ; they are vertical cavities one inch and a half in 



diameter, and twelve or more deep ; the animal holds fast by the expanded end of its foot. (S. P. 



Woodward.) Twenty-five species are known in Britain, Africa, Madeira, the Mediterranean, &c. 



FAMILY XX. MYACIDJE. 



The valves of the shell are gaping behind, opaque and strong, covered with a wrinkled 

 epidermis ; the mantle is almost closed ; the foot small ; the siphons united and retractile. 



Genus Mya, "Gaper." The shell is gaping at the ends; the left valve smaller than the right, with 

 a large process for the cartilage ; the siphonal fold is large ; the epidermis extends over and encloses the 

 siphons, which are partially retractile. Some of the species of Mya, as M. arenaria and M. truncata, 

 have a high northern range, being found through the Arctic seas; they are considered excellent 

 food, and are not only eaten by man, but by the walrus, the Arctic fox, and even by birds. Ten 



species are known living. 



* Greek, mesos, middle, and desmos, ligament. 



SOLEN VAGINA. 



