500 



LAMELLIBRANCHIATA. 



shaped. Taken as a whole, the Cyprinidce have passed their 

 acme, and have begun to decline in numbers and import- 

 ance. The family is one of great palseontological interest, 

 but only the more important forms belonging to it can be 

 here considered. 



The Cyprinidce fall naturally into two great groups, the 

 one represented by Cyprina and the other by Astarte, the 

 latter being sometimes raised to the rank of a distinct family 

 (Astartidw.) In Cyprinct (fig. 366, A and B) the shell is large, 

 strong, and rounded, with a thick epidermis, a strong ligament, 

 and two cardinal teeth in each valve, flanked by a single 

 lateral tooth. Numerous fossil species are known, commencing 

 in the Trias. Cypricardia (or Trapezium] has a trapezoidal 

 shell, usually radiately striated, the hinge with three radiat- 

 ing cardinal teeth. The true Cypricardice are probably Secon- 

 dary and Tertiary, several recent species being also known. 

 We may, however, place in its vicinity the Cypricardella of 

 the Carboniferous, the Gypricardinia of the Silurian, Devo- 

 nian, and Carboniferous, and the Gonioplwra of the two first 



Fig. 366. Types of Cyprinidce. A, Hinge of Cyprina tumida Pliocene ; B, Hinge of Cyprina 

 Saussuri Cretaceous ; c, Interior of Astarte detrita Jurassic ; D, Interior of Isocardia crassa 

 Pliocene. (After Pictet.) 



of these formations. Lastly, Isocardia (fig. 366, D), including 

 the recent " Heart-cockles," has a heart-shaped and inflated 

 shell, and the beaks remote and sub-spiral. The Isocardice 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. 



