LAMELLIBRANCHIATA. 



235 



Astarte includes thick, generally concentrically - furrowed 

 shells. Two hundred fossil species are known, commencing 

 in the Lias. 



Crassaiella (fig. 194) comprises thick, solid, ventricose 

 shells, attenuated posteriorly, and generally having a concen- 



Fig. 194. Crassatella ponderosa. Eocene Tertiary. 



trically-furrowed surface. Unlike the two preceding genera, 

 Crassatella has the ligament internal. The genus commences 

 in the Cretaceous Rocks, is abundant in the Tertiaries, and is 

 well represented at the present day. 



In the Heart-cockles (Isocardia) the beaks are remote and 

 sub-spiral, and the shell is heart-shaped. The Isocardia do 

 not appear to have existed in the Palaeozoic period, but com- 

 mence in the Trias, are tolerably abundant in the Oolites and 

 Cretaceous Rocks, decline in numbers in the Tertiaries, and 

 are represented by a few forms in existing seas. 



Cardita (fig. 195) includes Cockle-shaped shells, which 

 have radiating ribs, an external ligament, and a toothed 



rdita planicosta. Eocene Tertiary. 



margin. The genus commences in the Trias, but attains its 

 maximum in the Tertiary period, about a hundred species 

 having been enumerated from rocks of this age. 



