876 



MANUAL OF THE MOLI.USCA. 



Types, K. acuta, Ph XV., Fig. 18; furcillata, Fig. 10; 

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

 psittacea, Fig. 175 (p. 5, Fig. 4). 



Ventral aspect. tJmbonal aspect. 



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

 Fig. 176. TJmbonal 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. 



Shell trigonal, acutely beaked, usually plaited ; dorsal valve 

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

 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 palaeozoic 

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

 for a pedicle. Both the recent Rhynclionellai are black ; R. octo^ 

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

 America, Europe, Thibet, China. 



Sub-genera. ? Porambonites, Pander. P. aequirostris, 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. Ventral valve with converging dental plates {d) supported 



