CONCHACEA. 



737 



Goniophora (Silurian and Devonian) should find a place in the neigh- 

 bourhood of Cypricardia. 



Family 3. Venerid^e. — In this family the animal is free and 

 locomotive ; the mantle has a large anterior opening for. the foot ; 

 and respiratory siphons, which may be separate or more or less 

 united, are developed. The foot is tongue-shaped and compressed, 

 in some cases grooved and byssiferous. The shell (fig. 618) is 

 regular, suborbicular or oblong, equivalve, and furnished with an 

 external ligament. The hinge usually carries three diverging car- 

 dinal teeth, and the development of the lateral teeth is variable. 

 The pallial line usually shows a deeper or shallower sinus, but in 

 some cases is entire. The Veneridtz are all marine, and the family 

 includes some of the most highly organised and most beautifully 



Fig. 618. — Venus cincta, from the Miocene rocks of Austria. (After Zittel.) 



coloured examples of the entire class of the Lamellibranchiata. No 

 Palaeozoic representatives of the family have been as yet detected. 



In the genus Ve?ius (fig. 618) the shell varies greatly in form and 

 in surface-ornamentation, and is principally distinguished by its wide 

 hinge-plate, furnished in each valve with three diverging cardinal 

 teeth. The genus, in its typical form, appears to begin in the 

 Jurassic rocks, and the Tertiary rocks have yielded a large number 

 of forms, while about two hundred species are known to exist at the 

 present day. Very nearly related to Venus is the genus Cytherea 

 (Meretrix), which is largely represented at the present day, and has 

 numerous fossil forms, commencing in the Jurassic rocks. 



In the genus Dost?iia {Artemis) the shell is orbicular, compressed, 

 and concentrically-striated, with a deep " lunule " and a deep and 

 pointed pallial sinus (fig. 556, b). The earliest fossil forms appear 

 in the Cretaceous rocks. Venerupis, again, has the shell radiately 

 ribbed, and at the same time furnished with concentric ridges, the 

 general form of the shell being oblong. In Tapes, lastly, the shell 

 is oblong, with anteriorly placed beaks, a concentrically-striated 



vol. 1. 3 A 



