212 THE SEA SHORE 



receives its name from the yellow ground, which, is variously marked 

 by deeper tints ; another (T. decussata) is so called on account 

 of the cross grooves with which the shell is sculptured ; and a third 

 (T. virgineana), which inhabits the muddy bottoms of deep water, 

 is prettily marked by radiating bands that run from the umbones to 

 the ventral margins. 



We now come to the family Cyprinidce, in which the shell is 

 regular in form, oval or elongated ; and the valves, which are equal 

 in size, are thick and solid, and fit closely. The teeth are beauti- 

 fully formed, the central ones numbering from one to three in each 

 valve, and the pallial line is not sinuated. The mantle lobes are 

 united on the posterior side by means of a kind of curtain that is 

 pierced by two siphonal openings. There are two gills on each 

 side, united posteriorly, and the foot is tongue-shaped and thick. 



The typical genus Cyprina contains a large mollusc (C. islan- 

 dica), which is moderately common round our shores, especially in 

 the north, but is not often seen above low-water mark, except when 

 washed up by storms. The shell is oval and thick, with the 

 umbones prominent and turned towards the posterior side, and the 

 ligament is strong and prominent. It is entirely covered with a 

 thick epidermis, of a rich brown colour, often exhibiting a fine silky 

 gloss, especially near the margins. The interior of the shell is white, 

 and the adductor impressions oval and polished. 



The same family includes some smaller shells that inhabit deep 

 water, and are therefore not commonly seen on the beach. Among 

 these are two species of the genus Astarte, one of which is deeply 

 furrowed in a direction parallel with the margins ; also Circe 

 minima, which seldom exceeds half an inch in length. Although 

 so small compared with Cyprina, these shells may be identified by 

 their clothing of epidermis, together with the family characteristics 

 given above. 



The CyprinideB also contains the interesting Heart Cockle 

 (Isocardia cor), the form of which is so characteristic that identifica- 

 tion is easy. The heart-shaped shell is thick and strong, and is swollen 

 out in such a manner that the umbones are wide apart. These 

 latter are also curved into a spiral form, and the ligament between 

 them is prominent. The colour of the shell is variable, the 

 epidermis being of any shade from a yellow to a dark brown. The 

 foot is small and pointed, and the siphons fringed. 



The Heart Cockle burrows in sand by means of its foot, going 

 down just far enough to bury the whole of its shell, and always 



