66 PALEONTOLOGY 



The fresh-water Cycladidce are represented in the Wealden 

 and eocene by many species of Gyrena, mostly of small size. 

 The recent Corbicula fimninalis of eastern rivers is a common 

 fossil of the pliocene tertiary in England and Sicily. 



The Cyprinidce and Astartidce are more abundant as fossil 

 shells, and had a wider range of old than at the present day. 

 Nearly 100 species of Cyp^ina have been catalogued, com- 

 mencing in the trias ; the dentition of the older species is, 

 however, somewhat peculiar. The Isocardice are almost as 

 numerous, and have the same range, but many of the fossil 

 Isocardia-looking shells are really related to the Anatinidce. 

 A yet higher antiquity has been assigned to Gypricardia, a 

 genus now very scarce and difficult to obtain, on account of 

 its habit. The palaeozoic Pleuropliorus (fig. 13, 9) is dis- 

 tinguished by the prominent ridge behind the anterior 

 muscular impression ; and Megalodon (J. Sby.), by the plate 

 supporting the posterior adductor. This genus is represented 

 in the oolites by Pachyrisma (fig. 16, 1), and in the tertiaries 

 and modern seas by Oardilia. 



The genus Astarte, now limited to a dozen species in the 

 North Atlantic and Arctic seas, has an almost world-wide 

 geological distribution, and counts 200 species in d'Orbigny's 

 catalogue, commencing with the lias period. Crassatdla, now 

 almost a southern form, is common in the cretaceous and 

 tertiary strata of Europe. Closely allied to Astarte is the 

 extinct genus Opis (fig. 16, 3), of which there are 42 species 

 in the secondary series ; and Ga/rdinia (fig. 16, 2), characteristic 

 of the lias and oolites. The so-called Unios of the coal 

 measures (Anthracosia, King) are probably members <>f this 

 group. One hundred species of Gardita (including Vcixri- 

 cardia) are found in the secondary and tertiary strata ; of the 



* "They occur in the valuable layers of clay-ironstone called ' mussel- 

 bands,' associated with Nautili, Discinir, etc. In Derbyshire the mussel band 

 is wrought, like marble, into wises." — Woodward. 



