72% LAMELLIBRANCHIATA. 



the centre of the dorsal margin, the margins crenated, and the 

 pallial line more or less indented. The surface is adorned with 

 radial ribs or strise, which often carry spines. A very large number 

 of recent species of Cardium are known, and between three and 

 four hundred fossil forms have been referred here. Certain Palae- 

 ozoic types have been placed under this head, but the affinities of 

 these are doubtful. On the other hand, the genus is largely repre- 

 sented by undoubted types throughout the Mesozoic and Kainozoic 

 series. The genus Cardium has been broken up into numerous 

 minor groups, most of which can be with difficulty, or not at all, 

 recognised in the fossil condition. Papyridea, Lcevicardium, and 

 Lithocardium are Secondary to Recent types closely allied to Car- 

 dium proper. Protocardia (fig. 605) has the posterior slope of the 



shell radiately ribbed, while 

 the rest of the shell is con- 

 centrically striated. Hemi- 

 cardium has keeled valves, 

 the shell appearing cordate 

 as viewed from behind or in 

 front. Lymnocardium and 

 Adacna include brackish- 

 water and fresh-water Cockles, 

 in which the cardinal teeth 



Fig. 605. — Cardium {Protocardia) Hillanum. .. , .. 



Upper Greensand. are small or obsolete ; and 



the species of the former are 

 common in some of the fluviatile and estuarine deposits of the 

 Upper Tertiary period. The recent species of Adacna are found 

 abundantly in the Black Sea and Caspian Sea, and in the Sea of 

 Aral, often in quite brackish water, and the fossil species are extra- 

 ordinarily plentiful in the Tertiary deposits of Austria, Hungary, and 

 Southern Russia. The genus is remarkable for the exceeding vari- 

 ability of the hinge as regards the number of teeth. Byssocardium, 

 again, includes Eocene and Miocene Cockles, in which the shell is 

 truncated anteriorly, and possesses a large byssal sinus. 



The most remarkable of the early types of the CardiidcB is the 

 genus Conocai'dium ( = Pleurorhynchus), which ranges from the Or- 

 dovician to the Carboniferous, but is specially characteristic of the 

 Devonian and Carboniferous formations. The shell in this genus 

 (fig. 606) is keeled, and very oblique, the anterior end (sometimes 

 regarded as the posterior end) of the shell being short and abruptly 

 truncated, so as to appear as a cordate flattened area when the 

 shell is viewed from the front. Just below the beaks the shell is 

 produced anteriorly into a long cylindrical tubular projection or 

 beak. Posteriorly the shell is elongated and contracted, the valves 

 being widely deficient or gaping at the extremity. The hinge-line 



