CARDIUM. 287 



umbones central : lunule slight and indistinct : ligament short 

 but strong, horncolour, with sometimes a greenish tint, con- 

 tinued on the anterior side in large specimens: hinge-line 

 gently curved : hinge-plate thick and rather broad, reflected 

 on the posterior side, where it is usually stained with chocolate 

 colour, occupying about one-third of the circumference : teeth, 

 in the right valve one double cardinal, shaped like a reversed 

 V, and set obliquely, and a strong but short double lateral on 

 each side; inner teeth and Iamina3 much smaller and more 

 sunk than those next to the hinge-line ; in the left valve are 

 similar cardinal and lateral teeth, having their relative size and 

 position reversed, but these laterals are not so dispropor- 

 tionately small as those in the right valve : inside somewhat 

 glossy and nacreous, deeply stained with purplish-brown on 

 the posterior side, fluted towards the margin, which is armed 

 with short spikes, corresponding with the outside furrows : 

 pallial scar slight and wavy : muscular scars very strong, pos- 

 terior one the longer. L. 1-6. B. 1-7. 



Var. 1. rustica. Shell smaller and thinner, with the pos- 

 terior side usually more produced and wedge-shaped : colour 

 purplish-brown or light orange, often disposed in transverse 

 rows. C. rusticum, Chemn. Conch. Cab. vi. p. 201 (referring to 

 Linne's species of the same name), pi. 19. f. 197. 



Var. 2. crenulata. Shell oval and more equilateral ; front 

 margin nearly straight. C. crenulatum, Lam. An. sans Yert. vi. 

 p. 13. 



HABITAT : Gregarious everywhere in our sandy bays, 

 from low- water mark to a few fathoms. Var. 1. Estuaries 

 of brackish water. Var. 2. Plymouth (Reeve) ; Herm 

 (Lukis); Dogger Bank (Rich). This well-known spe- 

 cies occurs in all our pliocene tertiaries from the latest 

 raised beach to the Coralline Crag ; and I have found 

 it in upper miocene strata near Antibes. It is very 

 generally distributed through the North Atlantic Ocean, 

 from Iceland (Mohr) to the ^Egean (Forbes). The 

 variety rustica has also a range of similar extent ; and, 

 according to Middendorff, it inhabits the Aral Sea, 

 Caspian, Black Sea, and Russian Lapland. Lamarck 

 gives " La Manche " for the other variety ; and I am 



