876 MANUAL OF THE MOLLUSCA. 
Types, R. acuta, Pl. XV., Fig. 18; furcillata, Fig. 19; 
spinosa, Fig. 20; acuminata, Fig. 176; nigricans, Fig. 174; 
psittacea, Fig. 175 (p. 5, Fig. 4). 
Veniral aspect. Umbonal aspest. 
Fig. 176. Rh. acuminata, internal casts. 
Fig. 176. Umbonal aspect, with the dorsal valve above (Coll. Professor King). 
Ventral aspect (Coll. Professor Morris). A, adductor; R, cardinal; P, pedicle; V, 
vascular ; O, ovarian impressions. 
Shell trigonal, acutely beaked, usually plaited; dorsal valve 
elevated in front, depressed at the sides; ventral valve flattened, 
or hollowed along the centre, hinge-platessupporting two slender 
curved lamelle; 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 palzeozoic 
examples, the beak is so closely incuryed as to allow no space 
for a pedicle. Both the recent Rhynchonelle are black; FR. octo- 
plicata of the Chalk sometimes retains six dark spots. : 
Distribution, 4 species. LR. psittacea, Labrador (low water ?), 
Jiudson’s Bay (100 fathoms), Melville Island, Sitka, Icy Sea. 
it. nigricans, New Zealand, 19 fathoms. 
Fossil, 332 species. Lower Silurian—. North and South 
America, Europe, Thibet, China. 
Sub-genera. ? Porambcnites, Pander. P. sequirostris, Schl. 
Shell impunctate; surface minutely pitted; each valve witha 
minute hinge-area and indications of two septa; foramen 
angular, usually concealed. Distribution, 8 species. Lower 
Silurian. Russia and Portugal. 
Camarophoria, King. T. crumena, Martin (sp.). Figs. 177, 
178. Ventral yalye with conyerging dental plates (d) supported 
