CONCHIFERA. 447 
the deep and rugose cartilage pits: lower valve with a thick outer 
layer, ofton foliuceous; its cavity deep and straight, with two 
Fig. 241. Interior of lower valve. Fig. 242. Interior of upper valve. 
Radiolites mammillaris, Math. 4. L. Chalk. S. Mamest, Dordogne. 
é, ligamental inflection; m, pallial line; c, c, cartilage pits; a, a, adductor impressions 
and processes ; ¢, teeth and'dental sockets. 
cental sockets and lateral muscular impressions; upper valve 
‘flat or conical, with a central umbo; outer layer thin, radiated ; 
Fig. 243. Side views of the upper valve of R. mammillaris; 1, ligamental inflection 3 
t, teeth; a, a’, muscular processes. 
umbonal cayity inclined towards the ligament; teeth angular, 
striated, supporting,curved and sub-equal muscular processes. 
The upper valve of #. flewriausus 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 lamine. Specimens frequently 
occur 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 
