118 CRETACEOUS LAMELLIBRANCHIA. 
Deseviption.—Shell oval, oblique, higher than long. Right valve variable, 
flattened when attached by its entire surface, more convex when attached by a 
part only, the umbonal part sometimes much produced and talon-like. The 
attached part with concentric lamelle, 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.’ 
Measurements of left valve: 
(1) (2) (8) (4) (5) (6) (7) (8) (9) (20) (11) (12) (18) 
Length 5 BL Bul BHO) Xe), DY) Bes ys) BAL Ges) Bs} Wil as) LS} seawan, 
Height 5 Bio) Bi) ov! BY) Bil BS) By Bey Ws) Bs) Wes Wes) 1G) 
(1—9) Cambridge Greensand. 
(10—13) Red Limestone, Hunstanton. 
Affinities. —For the relation of S. gibbosus to S. Roemeri see p. 117. 
In 8S. Dutempleanus the shell is less oblique than in S. 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. giblosus are never so distinctly marked in WN. 
Dutempleanus. 
Remarks.—The specimens figured by d’Orbigny, as pointed out by Pictet and 
Campiche, are worn examples with the shell imperfect. The specimens found in 
the Cambridge Greensand 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 mght 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 ; they 
are generally regularly developed on the stronger ribs, but occasionally occur more 
irregularly (fig. 8 a). 
The specimens found in the Red Limestone of Hunstanton and Speeton are, on 
the average, smaller than those im the Cambridge Greensand, and the spies, 
probably owing to the hardness of the matrix and consequent difficulty of 
extraction, are usually wanting or indistinct, but one specimen in the British 
Museum shows them clearly. 
1 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. 
