36 PALEONTOLOGY 



but from them they can be distinguished by the im- 

 punctate shell, the surface of which, under a lens, com- 

 monly shows a silky fibrous appearance. At one time 

 all the species of the sub-order were included in the one 

 genus Rhynchonella, and although the Palaeozoic and 

 Triassic forms have been separated off into separate 



FIG. 8. PALAEOZOIC RIIYNCHONELLIDS. 



a, a', \Yihonia wilsoni (J. Sowerby), xf; b, b' , Pngnax acitminatus 

 (Martin), x J ; a, b, side-views ; a', b' , anterior view. 



genera, the sorting of the Jurassic and Cretaceous species 

 is only begun. 



Among Palaeozoic genera are Camarot&chia (Sil.-Carb.) 

 with a small spondylium and septum in the ventral valve ; 

 Wilsonia (Fig. 8, a ; Sil.) similar, but of more globose 

 shape and finer ribbing; Rhynchotreta (Sil.) with acumi- 

 nate beak on which the foramen encroaches, and con- 

 spicuous deltidial plates ; Pugnax (Dev.-Carb.) with very 

 strong fold and sinus, and few strong ribs or even smooth 

 as in the familiar Carboniferous Limestone species, 

 P. acuminatus (Fig. 8, b). 



