CARDIUM. 3 



An assemblage presenting so many attractions has, of 

 course, engaged considerable attention, and our knowledge 

 of the extent and geographic range of the genus is conse- 

 quently very advanced. There are, probably, very nearly 

 two hundred species of Card'mm in existence, and the 

 localities of considerably more than half are well known. 

 We find the great central assemblage of Cockles in the In- 

 dian ocean, a region where about a third of the species 

 are congregated. Around this centre the number of specific 

 forms diminishes, though found in every sea. They are most 

 plentiful everywhere within the tropics, and diminish as 

 we proceed northwards and southwards ; but some of the 

 forms most prolific in individuals, and most gregarious in 

 habit, are present in cold climates, and make up by abun- 

 dance for the absence of variety. Of these, several are 

 valuable articles of food ; and it may be said of all the 

 Cardia that they hold a high rank among mollusca both 

 for nutritive qualities and excellence of flavour. The genus 

 contains several remarkable abnormal forms ; some of the 

 most singular are to be found in the Caspian and other 

 relics of the great Aralo-Caspian Sea, — the demonstra- 

 tion of Avhich mighty inland ocean is among the finest dis- 

 coveries of Sir Roderick Murchison. 



The geological distribution of this interesting group 

 corresponds in extent with the geographical. Even in 

 Palaeozoic strata we find the fossilized remains of mollusks 

 closely allied, if not belonging to Card'mm. In the secondary 

 rocks, even in their oldest members, well-marked forms of 

 Cardium are not unfrequent, often singularly simulating 

 those of existing times. During the later part of the 

 secondary epoch and the beginning of the tertiary a group 

 of half-ribbed Cockles, seemed to have been developed at 

 the expense of ordinary forms, and to have dwindled away 



