286 



MANUAL OF THE MOLLUSCA. 



structure ; 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; upper 

 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. fleuriausus has an oblique umbo, with a distincl 

 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 laminee. Specimens frequently occur in which the outer she! 

 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 deepl} 

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

 valve. 



