222 



MOLLUSCA. LAMELLIBRANCHIA 



adductor. The small valve is formed of two layers ; the outer is 

 thin and prismatic, the inner is porcellanous and traversed by 

 numerous canals. The outer layer of the large valve is formed of 

 small prisms arranged in parallel layers obliquely to the surface of 

 the shell ; the inner is porcellanous and formed of thin leaflets. 

 Upper Cretaceous. Ex. H. comu-vaccinum. 



Fig. 96. Fig. 97. 



Fig. 96. Transverse section of the large valve of Hippurites comu- 

 vaccinum. r, umbonal cavity; e, internal layer of shell; d, external 

 layer; I, m, n, folds; t, cardinal teeth; a, anterior adductor; 

 a', posterior adductor ; c, cavity ; c', cardinal fossa. Cretaceous. 

 (From Woodward.) x ^. 



Fig. 97. Longitudinal section of the small valve and part of the large 

 valve of Hippurites comu-vaccinum. u, umbonal cavity of small 

 valve; d, external layer of shell; r, internal layer; i, part of cavity 

 between the valves; a, anterior adductor; a', posterior adductor; 

 t, t' anterior and posterior cardinal teeth of small valve ; I, cardinal 

 tooth of large valve. (From Woodward.) x |. 



Radiolites. Shell large, thick, valves very unequal. The 

 large (loiver) valve conical or sub-cylindrical, generally straight, 

 fixed by its apex (umbo) ; surface with vertical ribs, and thick, 

 horizontal projecting layers which are more or less regularly folded ; 

 with a ligamental fold extending from the apex to the margin, and 

 two vertical undulations corresponding to the positions of the anal 

 and branchial orifices ; outer layer of shell very thick, formed of 

 polygonal or prismatic cells ; inner layer thin, porcellanous, often 



