LAMELL1BRANCHIATA 65 



muscular plate, the other (or third) apparently a siphonal 

 inflexion like that in Trigonia and Leda (fig. 15, 4). 



The cockle-shells (Cardiadai), as they have a world-wide 

 distribution now, had a corresponding range in time, and are- 

 found in all strata from the Silurian upwards. The com- 

 monest fossil type of Cardium is ribbed concentrically on the 

 sides, and radiately on the posterior slope, a style of ornament 

 almost unique amongst the 200 recent species. The Caspian 

 cockles, distinguished by a sinus in the pallial line, appear to 

 have inhabited the Aralo-Caspian region almost from the 

 middle tertiary period ; the hinge-teeth are reduced to one 

 (Monodacna) or two (Didacnci) in each valve, and are some- 

 times quite wanting even in the young shell (Adacna, Eichw.) 

 Lithocardium aviculare (fig. 1G, 7) is a characteristic shell of 

 the Paris basin, and appears to have spun a byssus, like the 

 fry of some recent cockles ; it also resembles the oriental 

 Tridacna, of which a species is found in the miocene of 

 Poland. The genus Conocardium (fig. 13, 8) of the upper 

 Silurian and carboniferous systems is remarkable for the 

 prismatic cellular structure of its shell, and the truncation of 

 the posterior (T) side of the valves, which are furnished in 

 some species with a slender siphonal process. 



The Lucinidce, allied to the cockles in their hinge-structure, 

 are also plentiful in the fossil state, and have as wide a range. 

 They are usually recognisable, even when in the condition of 

 internal casts, by their circular form and the oblique ridge on 

 their disk. Casts of Lucina also exhibit the peculiar narrow 

 outline of the anterior adductor detached from the pallial line. 

 Cryptodon, Diplodonta, Kellia, and PytMna are found in the 

 eocene tertiary. Cordis, under the sub-generic form of Sphcei a, 

 commences in the trias ; another modification, found in the 

 oolites and chalk (Unicardium, d'Orb.), is edentulous; and 

 Tancrcdia (Lycett), a compressed triangular shell, with a 

 dentition like Coring is frequenl in the lias and oolite. 



F 



