88 CONCHIFERA. DIMYAIKA. 



a hollow in the opposite valve ; two distant, lateral muscular 

 impressions. 



The shells of this genus are generally found at no great depth. They are always 

 found attached by their larger valve to rocks, or corals, or grouped together in various 

 forms. 



* Beaks turning from left to right. 



C. Lazarus, Lin. Shell imbricated ; lamellae dilated and waved, sub- 

 lobate, obsoletely striated ; colour whitish, red, or yellow. 2 in- 

 ches in diameter. American seas Favanne, pi. 43, fig. A 1, and A2. 



C. gryphoides, Lin. Shell imbricated, with adpressed, short, some- 

 what vaulted scales j pale rose or citron coloured, mottled with 

 white. 2 inches in diameter. Inhabits Mediterranean sea. 

 Poll, Test. ii. pi. 23, fig. 3. 



* Beaks turning from right to left. 



C. arcinella, Lin. Shell somewhat heart-shaped, with spinous lon- 

 gitudinal ribs, and excavated dots ; posterior depression cordiform ; 

 colour white, sometimes tinged with rose colour j inside yellow- 

 ish. 1^ inch long. Inhabits S. American seas. Chem. vii. pi. 

 52, fig. 522, 523. 



Fossil shells of this genus are found on the Continent in France and Italy, and in 

 Britain in the oolite, green-sand, and London clay. 



Gen. 3. DICERAS, Lam. Chama, Brug. 

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

 ing in an irregular spiral form ; a very large, thick, concave 

 tooth in the larger valve; two muscular impressions. (Fossil.) 



D. arietina, Lam An. Mus. vi. pi. 55, fig. 2, a, b. 



SECTION II. LAMELLIPEDES. 



FAMILY I. NAYADES. 



Fluviatile shells, of which the hinge has sometimes an irregular, 

 simple, or divided tooth, and a longitudinal prolonged one ; 

 sometimes none at all ; or is furnished in all its length with 

 irregular granular tubercles ; posterior muscular impression 

 compound ; umbo often decorticated. 



This family is composed of shells which are found in rivers, ponds, and lakes. 

 Their shell is free, regular, equivalve, always transverse, and furnished with a brown, 

 ish green epidermis, which is generally worn off on the umbones. The muscular 

 impressions are lateral and distant, and that on the posterior side is composed of 

 two or three distinct, unequal impressions. The animal has no projecting tube or 

 syphon. The foot is elongated, transverse, and rounded, which it projects between the 

 valves, and uses to shift its position. The shell is always partly sunk in the mud. 



Gen. 4. IRIDINA, Lam. 

 Shell equivalve, inequilateral, transverse, with small recurved 



almost straight beaks ; hinge long, linear, attenuated towards 



the middle, tuberculous, almost crenated ; tubercles unequal ; 



ligament exterior, marginal. 

 I. exotica, Lam. Shell transversely oblong, with longitudinal very 



fine striae ; colour reddish, brilliant silvery, especially in the in- 



