376 



MANUAX OF THE MOLLTJSCA. 



Typos, E. acuta, PI. XY., Pig. 18; furcillata, Fig. 19; 

 spinosa, Fig. 20; acuminata, Fig. 176; nigricans, Fig. 174; 

 psittacea, Fig. 175 (p. o. Fig. 4). 



Ventral aspect. Umbonal aspeot. 



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

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

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

 vascular ; 0, ovarian impressions. 



Shdl trigonal, acutely beaked, usually plaited ; dorsal yalv^ 

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

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

 curved lamellce ; 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 palaeozoic 

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

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

 plicata of the Chalk sometimes retains six dark spots. 



JDistribut ion, 4: s-pccies. i?. psiYtocea, Labrador (low water?), 

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

 R. nigricans, New Zealand, 19 fathoms. 



Fossil, 332 species. Lower Silurian — . North and South 

 America, Europe, Thibet, China. 



Sub-genera. ? Poramhcnites, Pander. P. requirostris, Schl. 

 Shell impunctate ; surface minutely pitted; each valve with a 

 minute hinge-area and indications of two septa; foramen 

 angular, usually concealed. Distrihution, 8 species. Ijower 

 Silurian. Eussia and Portugal. 



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

 178. Yentral valve with converging dental plates {d) suj^portcd 



