Lamarck's Genera of Shells. 27 



spects they are very similar. The shell is rather thick, brilliant 

 pearly, reddish, especially internally, and iridescent. 



One species. Iridina exotica*. 



Shell transversely oblong, longitudinally striated ; striae very 

 delicate ; lateral edges rounded ; beaks slightly projecting above 

 the hinge. Rivers of Hot Climates. PI. I. Fig. 72. 



10th. Family. 

 CiiAMACEA. (3 Genera.) 



Shell inequivalve, irregular, fixed. Hinge with one thick 

 tooth, or none at all. Two separate, lateral, muscular impres- 

 sions. 



The ligament of the shells belonging to this family is exter- 

 nal, and sometimes sunk irregularly towards the interior ; with 

 respect to the hinge, they have some analogy to the tridacnea ; 

 they are often lamellar and spinous, their beaks always irregu- 

 lar, sometimes large and contorted. The animal has only 

 short, disunited syphons. The shells are attached to rocks, 

 corals, and often to each other. 



1. Dicerasf. 



Shell inequivalve, adhering; beaks conical, very large, di- 

 verging, irregularly spiral. One very large, thick, concave, 

 subauricular, prominent tooth, in the largest valve. Two mus- 

 cular impressions. 



The diceras somewhat resembles the isocordia in external 



• Exotic. We have given the characters of /. exotica, as being the only 

 species described or named by Lamarck. Mr. Sowerby {Genera of Recent 

 and Fossil Shells,) has a beautiful figure of another species, /. elongata ; and 

 Mr. SwiimsoD, {Phil. Mag. Ixi. 112,) has described three species, «z., /. 

 striata, I. elongata, and /. ovata. Our figure is taken from the single polished 

 valve, in the British Museum, which Mr. Swainson thinks probably belongs 

 to the last species, if properly to either of them. Jt was certainly a mis- 

 take, as he observes, to call it /. exotica. Mr. Swainson describes the 

 I. ovata, as follows. " Shell smooth, transversely oval ; umboues promi- 

 nent and nearly medial." 



t From $i( and xi^a;, signifying with two boms :' 



