286 



MANUAL OP THE MOLLUSCA. 



structiu-e ; internal margins smooth or finely striated, simple, continuous ; 

 ligamental inflection very narrow, dividing the deep and rugose cartilage 

 pits : lower valve with a thick outer layer, often foliaceous ; its cavity deep 

 and straight, with two dental sockets and lateral muscular impressions; wpiper 

 valve flat or conical, with a central umbo ; outer layer thin, radiated ; um- 

 bonal cavity inclined towards the ligament ; teeth angular, striated, supporting 

 curved and sub-equal muscular processes. 



The upper valve of R. fleiiriausus has an oblique umbo, with a distinct 

 ligamental groove. The foliations of the lower valve are frequently undu- 

 lated ; they are sometimes as thin as paper and several inches wide. 



The umbonal cavity of the lower valve is partitioned off by very delicate 

 funnel-shaped laminaj. Specimens frequently occm* in which the outer shell 

 layer is preserved, whilst the inner is wanting, and the mould ("birostrites") 

 remains loose in the centre. The interior of the outer shell layer is deeply- 

 grooved with lines of growth, and exhibits a distinct ligamental ridge in each 

 valve. 



Fig. 205. Upper view. Fig. 206. Side view. 



Internal mould oi R. Hceninghausii, Desm. g. Chalk. 



u, umbo of left valve; r, right umbo; I, ligamental groove; c,c, cartilage; a, anterior 



adductor muscle ; a', posterior. 



In aged examples of R. calceoloides the ligamental inflection is concealed, 

 the cartilage pits partially filled up and smoothed, and the teeth and apophyses 

 so firmly wedged into their respective cavities, as to suggest the notion that 



at the will of the animal. 



Fossil, 42 sp. Neocomian — Chalk. Texas ; Brit. France, Bohemia, 

 Saxony, Portugal, Algeria, Egypt. 



Sub-genus ? Bi-radioUtes, D'Orb. R. caualiculatus, (Fig. 186, upper 

 valve) . Ligamental groove visible in one or both valves, sometimes occupy- 

 ing the crest of a ridge, and bordered by two similar areas, {a, a.) Fossil, 

 3 sp. Chalk, France. 



