876 



MAKtlAL OF THE MOLLtJSCJ 



Types, B* acuta, PI. XY., Fig. 18; furcillata, Pig. 19? 

 spinosa, Pig. 20 ; acuminata, Eig. 176 ; nigricans, Eig. 174 J 

 psittacea, Fig. 175 Qj. 5, Fig. 4). 



Ventral aspect* Umbonal aspeot. 



Fig. 1 76. Rh. acuminata, internal casts. 

 Fig. 176. Umbonal aspect, with the dorsal valve above (GoU. Professor King). 

 Ventral aspect (Coll Professor Morris). A, adductor; E, cardinal; P, pedicle; V, 

 Vasculai' ; 0, ovarian impressions. 



Shell trigonal, acutely beaked, usually plaited ; dorsal valye 

 elevated in front, depressed at the sides; ventral valve ilattened, 

 or hollowed along the centre, hinge-plates supporting two slender 

 curved lamellae ; dental plates diverging. 



The foramen is at first only an angular notch in the hinge- 

 line of the ventral valve, but the growth of the deltidium usually 

 renders it complete in the adult shell ; in the cretaceous species 

 it is tubular. In R. acuminata and many other palseozoic 

 examples, the beak is so closely incurved as to allow no space 

 for a pedicle. Both the recent Rliynchonellce are black ; R, odo- 

 plicata of the Chalk sometimes retains six dark spots. 



Distribution, 4 species. R. psittacea, Labrador (low water :), 

 Hudson's Bay (100 fathoms), Melville Island, Sitka, Icy Sea. 

 R. nigricans, New Zealand, 19 fathoms. 



Fossil, 3.32 species. Lower Silurian — . North and South 

 America, Europe, Thibet, China. 



Suh-genera. ? JPoramhonites, Pander. P. sequirostrisj SchL 

 Shell impunctate ; surface minutely pitted ; each valve with a 

 minute hinge-area and indications of two septa; foramen 

 angular, usually concealed. Distribution, 8 species. Lower 

 Silurian. Eussia and Portugal. 



Camarophoria, King. T. crumena, Martin (sp.). Figs. 177, 

 178. Yentral valve with converging dental plates {d) supported 



