CARDIUM. MACTRA. 23 



of the hinge mostly three, the lateral ones smooth, 

 in one valve. Two or three small species are 

 common on our coasts. I should also observe 

 that there is a convex fold on one valve and a 

 concave fold upon the other. Many of the 

 Teltina genus are found buried in the sea-sands. 



" Cardium. Generic character : shell equi- 

 valve, convex, ribbed, striated, or grooved, the 

 margin toothed : hinge with two teeth near the 

 beak, and a lateral one on each side : fifty-four 

 species. 



" Observe how the beaks of this common C«r- 

 dium, cockle, turn inwards, and the bosses pro- 

 ject. Another striking character is the ribs, that 

 are generally longitudinal, and not concentric or 

 transverse, as in Telhna, and, as you will see, 

 in Venus. C. aculeatum has small spines on the 

 valves ; C. costatum^ the ribbed cockle, is one of 

 the finest species of this genus, and C. cardissa 

 is a beautiful shell. (Plate 4.) The common 

 cockle is Cardizcm edide. 



Mactra. Generic character : shell bivalve, 

 unequal sided, middle tooth of the hinge compli- 

 cated, with a small hollow on each side, and lateral 

 side-teeth : thirty-seven species. 



^' The shells of this genus are usually thin, 

 brittle, and remarkably light. Mactra stidtorum 

 is a common species. (Plate 4.) 



'' DonaoG. Mar^rin of the shell often crenulate, 



