HEART URCHINS. 73 



group, the genus Lucina, there is less uniformity in 

 the hinge than usual ; and here we gladly have re- 

 course to the impression of the adductor muscles, one 

 of which, in this genus, is prolonged in a remarkable 

 manner. The impression of the mantle appears also to 

 afford excellent generic characters, though it has only 

 recently been admitted by conchologists into their 

 descriptions. It may be observed, on the inside of each 

 valve, forming a narrow line, more glossy than the rest 

 of the shell, connecting one muscular impression with 

 the other. It forms different curves in different genera, 

 and exhibits many minor variations. But our limits 

 do not permit us to consider the niceties of classifi- 

 cation, and we must refer for further information on 

 the subject to Messrs. Forbes and Hanley's History of 

 the British Mollusca. 



Much lower in the scale of being than Bivalve 

 Mollusca, but elaborately organized, and offering many 

 interesting points in their history, are the Heart 

 Urchins, a tribe of animals enclosed in egg-like shells, 

 coated with spines, which inhabit all our sandy bays- 

 There are several recent British species, but I shall 

 only mention the common Heart Urchin (Amphi- 

 dotus cordatus), Mermaid's Head, or Sea Egg, as it is 

 variously called, which is found all round the coast. 

 When alive, it is thickly clothed with fine hair-like 

 spines, each of which is articulated at base with a mi- 

 nute nipple, forming a ball-and-socket joint, so that 

 the spine can move freely in all directions. The spines 

 are of different forms and length on different parts of 

 the body, and, frail as they appear, serve the purpose to 



