Heart-Cockle and Fresh-water Cockle 99 



named. The white shell shows tlirough the thin 

 epidermis. The tooth-like folds of the hinge-plate 

 are stronger than in either of the other species. It is 

 a rare species, and appears to be restricted (in Britain) 

 to Skye and Shetland, where it occurs in deep water, 

 between 45 and 85 fathoms. 



The Round Double-tooth {Diiilodonict rotundata) is 

 almost round in outline, somewhat glossy, with 

 irregular concentric ridges; white, but covered in 

 front by the j^ellowish epidermis. There are two car- 

 dinals and two laterals in each valve, one of the 

 cardinals in each being cleft in order to receive the 

 undivided one of the opposite valve. The animal is 

 white, with large free lips, and the mantle edges are 

 united, save for the large aperture on the lower margin 

 where the foot emerges, and for the incurrent and 

 excurrent orifices on the lower margin. The shell is 

 about 1 inch long, and almost as deep from the beak 

 to the lower margin. It is found on the surface of 

 sandy mud in from. 12 to 20 fathoms, chiefly on our 

 southern shores, but also locally on the west, as at 

 Lundy, Anglesea, and Fishguard. 



There are six little bivalves of local occurrence 

 that have been named after Colonel Montagu, who 

 first discovered and described them. His name in 

 the Latinised form of Montacuta stands for that of 

 the genus. As they have no popular name, we think 

 it fitting to call them Montagu-shells. Montagu's 

 Urchin-shell {M. sidjstriata) is a small, yellowish, 

 oval, semi-transparent shell, about a quarter of an 

 inch in length. Its valves are marked b}^ about a 

 dozen faint ribs radiating from the beaks, but not 

 reaching to the front margin. The animal spins a 



