90 CONCHIFEHA. DIMYAIHA. 



decorticated; posterior muscular impression compound; hinge 

 with two teeth in each valve, the one short, irregular, simple, 

 or divided into two, substriated ; the other elongated, com- 

 pressed, lateral ; ligament exterior. 



The shell in this genus is covered with a greenish or brown epidermis, below which 

 it is in general nacreous or silvery. The beaks are always decorticated and ca- 

 rious. They inhabit fresh waters in both hemispheres, and many of them afford 

 pearls. 



* Primary tooth short and thick. 



U. elongata, Lam. (My a margaritifera, Lin.) Shell ovate-oblong, 

 compressed on the fore part, strong, ponderous, and coated with 

 a thick black epidermis ; hinge strong ; the primary tooth conical, 

 locking into a bifurcate one in the opposite valve ; beaks decor- 

 ticated ; inside pearly. 5 to 6 inches broad, and 2^ long. In- 

 habits mountainous rivers in Northern Europe. B Penn. Brit. 



Zoo/, iv. pi. 46, fig. 2. 



This species often produces pearls of considerable size and value. The moun- 

 tainous rivers of Scotland are periodically fished for the purpose of procuring them ; 

 and it is said that the pearls in the Scottish crown are chiefly native ones, from the 

 river Tay. 



U. purpurata, Lam. Shell oval-elliptic, tumid ; violet green, tinged 

 with purple; lateral tooth crenated. 5 i inches broad. Inhabits 

 large rivers of Africa Lam. vi. 1, 72- . 



U. ovata, Lam. Shell oval, subtumid, gaping laterally ; epidermis 

 yellowish ; umbones prominent ; striae almost lamellar. 3 inches 

 broad. Inhabits rivers in North America Lam. vi. 1, 75. 



** Primary tooth short, compressed, often crested. 



U. pictorum, Lam. Shell ovate-oblong, with a crenated primary 

 tooth, besides a long lateral tooth in one valve, received into a 

 groove in the other ; colour dusky green, with concentric wrin- 

 kles ; inside silvery. 3 inches broad, and 1 \ long. Inhabits ri- 

 vers in Europe. B. Browns Illust. pi. 26, tig. 2. 



U. Batava, Lam. Shell ovate, tumid, radiated with yellowish- 

 green ; rounded at both ends. 2 inches broad, and 1 inch long. 

 Inhabits rivers in Europe. B Brown's Illust. pi. 26, tig. 3. 



FAMILY II. TRIGONACEA. 



Primary teeth lamelliform, and striated transversely. 

 Gen. 8. CASTALIA, Lam. 



Shell equivalve, inequilateral, trigonal, with the beaks decorti- 

 cated and recurved posteriorly ; hinge with two lamellar 

 teeth, the posterior one remote, shortened, subtrilamellar ; the 

 other anterior, elongated, and lateral ; ligament exterior. 



C. ambigua, Lam. Shell oval, trigonal, tumid, flattened, and cor- 

 diform anteriorly, with longitudinal ribs transversely striated ; 

 epidermis brown, and the margin entire ; inside silvery. 1^ 

 inch broad. Lam. vi. 1, 67- 



