118 CRETACEOUS LAMELLIBRANCHIA. 



Description. — Shell oval, oblique, higlier than long. Right valve variable, 

 flattened when attached by its entire surface, more convex when attached by a 

 part only, the uml:>onal part sometimes much produced and talon-like. The 

 attached part with concentric lamellae, the free part with many radial ribs without 

 spines. 



Left valve moderately convex ; umbo more or less produced ; ribs numerous, 

 unequal : the stronger occur at regular intervals, and are separated by two or three 

 (rarely one only, or more than three) smaller ribs ; the stronger ribs bear numerous 

 spiny processes, the smaller ribs are usually without spines.^ 



Measnremenfft of h'ff oa/oe: 



(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) 



Length . 31 31 30 29 2(3 25 25 21 19-5 23 21 19 16 mm. 

 Height . 36 35 34 36 31 28 27 25 23 25 23 22-5 19 „ 



(1 — 9) Cambridge Greensand. 



(10 — 13) Eed Limestone, Hunstanton. 



Affinities. — For the relation of S. gibbosus to S. Boemeri see p. 117. 



In S. Didempleaiins the shell is less oblique than in 8. gibbosus ; the spines on 

 the left valve are generally more irregular and more nearly vertical than in the 

 latter (judging from the figure of Pictet and Campiche). The stronger ribs which 

 occur at regular intervals in S. gibbosus are never so distinctly marked in >S'. 

 Dnfempleanns. 



Remarks. — The specimens figui'ed by d'Orbigny, as pointed out by Pictet and 

 Campiche, are worn examples with the shell imperfect. The specimens found in 

 the Cambridge Grreensand vary considerably ; a gradual passage can be traced from 

 forms in which the right valve is flat (fig. 9 b) to others in which it is much pro- 

 duced and talon-like (fig. 11) ; in the right valve the appearance of the surface 

 depends largely on the amount of wearing the shell has undergone, — one specimen, 

 in which the ribs appear almost equal and without spines, agrees perfectly with 

 d'Orbigny's fig. 2 ; in most cases, however, the stronger ribs occurring at regular 

 intervals are distinctly seen (fig. 6 a). Only the bases of the spines remain ; tlie}^ 

 are generally regularly developed on the stronger ribs, but occasionally occui- more 

 irregularly (fig. 8 a). 



The specimens found in the Red Limestone of Hunstanton and Speeton are, on 

 the average, smaller than those in the Cambridge Greensand, and the spines, 

 probably owing to the hardness of the matrix and consequent difficulty of 

 extraction, are usually wanting or indistinct, Init one specimen in the British 

 Museum shows them clearly. 



^ The specimen figured by Pictet and Campiche shows spines on the small ribs. The general 

 absence of small spines on our specimens is probably due to the worn nature of the shell. 



