BIVALVES. — CARDIUM. 43 



adorned with longitudinal ridges and grooves, crossed by 

 transverse striae, similar to the common cockle, only much 

 more articulate. In others, again, the ridges are beset 

 with rows of acute spines, as in the C. aculeatum and C. 

 echinatum; but the exteriors of some have a perfectly 

 smooth and polished surface, as the egg-cockle, &c. 



The C. fragum and C. unedo exhibit a peculiar formation, 

 being sub-angular, and only heart-shaped when seen in a 

 particular position. A similar coincidence is observable in 

 the C. retusum, though, in other respects, it differs; for the 

 C. fragum and C. unedo have their ridges covered with 

 crowded pink or yellow elevated hmules or crescents, but 

 the C. retusum has nodules rather than imbricated scales. 

 The interior margin of this genus is almost universally 

 crenate or toothed. 



The hinge is furnished with two teeth, and a larger re- 

 mote lateral tooth on each side of it, each locking into its 

 dpposite. 



The C. edule, or common cockle, is found in great abun- 

 dance beneath the surface on sandy coasts; the fish affords 

 a wholesome and nourishing food. 



The most rare and valuable species of the genus Cardium 

 is the C. costatum or pipe-ridged cockle, which has rows 

 of white hollow elevated ribs, situated at regular distances 

 on its surface, and proceeding in a longitudinal direction 

 from the beaks to the margin; the interstices, or spaces be- 

 tween these ribs, are (in perfect specimens) of a fine dark- 

 brown colour, which gives the shell a great boldness of 

 character. 



A great part of this genus inhabits the European and 

 Northern seas; many are collected from the American, 

 African, and Indian oceans; the Mediterranean, likewise, 

 produces some of the species ; and the mouths of rivers, as 

 the Tees, Thames, &c. also supply specimens, though 

 rarely. 



