PHOLAS. P1DDOCK. 145 



much resembles the former, is known by the want of 

 cells on the back, by being furnished with only one lan- 

 cet-shaped dorsal valve, and more especially by the rounded 

 tubercles placed on the inner-margin of the hinge. These 

 tubercles, from some misunderstanding of the exact mean- 

 ing of Montagu, have been represented as supporting the 

 teeth : " Cardinis dente ex tuberculo orto," Maton and 

 Rackett. " Hinge with a tooth springing from a tubercle," 

 Wood* " The teeth of the hinge issuing from a tubercle/* 

 Dittwyn. The teeth, however, are inserted under the inner 

 margin of the hinge, as in the other species, and have no 

 attachment or connexion whatever with the tubercles, 

 which are seated on the centre of the margin and remote 

 from the insertion of the teeth. Ph. Candida is a much more 

 delicate and transparent species, rounded at the top and 

 without the sloping beaks, in consequence of which it has 

 very little frontal gape when closed ; the transverse stria 

 are more regular and at remoter distances, visible on the in- 

 side, and not so strongly crossed by the longitudinal ones, 

 by which means it is not so much echinated or spinous ; 

 the back, with its single dorsal valve and want of cells, rs 

 formed like Ph. parva; and its characteristic mark, inde- 

 pendent of its outline, is a single sharp curved supple- 

 mental tooth seated on the middle of the hinge, in the left 

 hand valve only, and locking into a groove of the opposite 

 valve. Of this strong specific mark, neither Maton and 

 Rackett nor Dillvvyn have taken any iSOtice ; and Mr. Wood, 

 in whose left hand figure this tooth is remarkably well re- 

 presented, calls it a folded process, and asserts that it is 

 found in both valves. 



The philosophy of their natural history may probably be 

 of no very difficult solution. The rock in which they are 

 imbedded is a cementation of the finest sand and lime, of 

 o very soi't a substance when first taken from its bed, as to 

 be easily cut with a knife into any form-, and sufficiently 

 absorbent to afford moisture for the purposes of life and 

 their peculiar action. The animals themselves abundantly 

 secrete a mild phosphoric solution, as may be seen by its 

 illuminating in the dark whatever is moistened wkh it, suf- 

 ficiently powerful to decompose the rock by the slow con* 

 of their gradually increasing, bulk. The atmospheric 



