60 CRETACEOUS LAMELLIBRANCHIA. 
C. Marceana (d’Orbigny), from the Cenomanian of Mans, is more oval in 
outline, the area and hinge-line are relatively shorter, and the surface of the shell 
is smoother than in C. glabra, Sowerby. 
According to Geinitz C. glabra occurs in the Lower Quadersandstone (Ceno- 
manian) of Koschiitz, Neiderschéna, and Goldenen Hohe (Saxony). All the 
specimens obtained are apparently internal casts, so that comparison is very 
difficult, but they appear to be relatively longer and more oblique than C, glabra, 
Sowerby, and the umbo is sharper than in wax casts taken from Blackdown 
specimens. ‘The same remarks apply to the Bohemian form referred by Reuss to 
this species. 
Cucullea (Idonearca) Shumardi, Meek and Hayden,’ from the Fox Hill Group 
of Upper Missouri, appears to be very closely related to C. glabra, but without 
seeing specimens I am unable to state whether it is really distinct or not. 
Remarks.—This is one of the commonest fossils in the Blackdown Greensand. 
In old specimens the radial ribs usually become obliterated, and the shell is 
then nearly smooth save for lines of growth. The appearance of the ornamenta- 
tion varies considerably according to the state of preservation of the shell. In 
old forms the shell becomes, as a rule, relatively longer, more convex, and more 
oblique. Before the adult state is reached the lateral teeth are placed obliquely 
to the hinge-line (Pl. XI, figs. 10,11). Two forms of this ‘‘species” may be 
recognised :—(i) in which the shell is rhomboidal in outline, rather compressed, 
and with a sharp carina (Pl. XII, figs. 3, 4); (i) in which the shell is of more 
rounded outline, more inflated, and has only an indistinct carina (Pl. XI, fig. 8). 
These two types can be seen in various stages of growth, but there are also some 
examples which seem to be intermediate in character. 
Types.—\ have not seen the type, but the specimens figured by Sowerby as 
C. glabra, C. fibrosa, and CO. carinata (all from Blackdown) are in the British 
Museum. 
Distribution.—Blackdown Greensand (especially zone x). Gault of Folkestone. 
1 «Proe. Acad. Nat. Sci. Philad.,’ vol. viii (1856), p. 86; Meek, ‘Invert. Cret. and Tert. Foss. 
U. Missouri’ (‘Rep. U.S. Geol. Surv. Territ.,’ vol. ix, 1876), p. 86, pl. xxviii, figs. 15 a—g; 
pl. xxix, fig. 4. 
