CONCSIFERA. 



447 



the deep and rugose cartilage pits : lower valve with a thick outer 

 layer, often foliaceous ; its cavity deep and straight, with two 



Fig. 241. Interior of lower valve. Fig- 242. Interior of upper valve. 



Rndiolites mammillaris, Math. |. L, Chalk. S. Mamest, Dordogne. 



^ ligamental inflection; m, pallia! line ; c, c, cartilage pits; a, a, adductor impressions 



and processes J t, teeth and'dental sockets. 



dental sockets and lateral muscular impressions; tipper valve 

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



'^ Hi^ I 



W 



Fig. 243. Side views of the upper valve of B. mammiUaris ; I, ligarnentai inflection 5 

 i, teeth ; a, a', muscular processes. 



umbonal cavity inclined towards the^ ligament ; teeth angular, 

 striated, supporting.curved and sub-equal muscular processes. 



The upper valve of i?. fleuriausus has an oblique umbo, with 

 a distinct ligamental groove. The* foliations of the lower valve 

 are frequently undulated ; 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 laminae. Specimens frequently 

 occur in which the outer shell layer is preserved, whilst the 

 inner is wanting, and the mould ("birostrites") remains loose 

 *n the centre. The interior of the outer shell layer is deeply 



