60 



CKKTACEOUS LAMKLLIBRANCHIA. 



G. Mavceana (d'Orbigny), from tlie Cenomanian of Mans, i- more oval in 

 outline, the area and hinge-line are relatively shorter, and the surface oi the shell 

 is smoother than in C glabra, Sowerby. 



According to Geinitz 0. glabra occurs in the Lower Quadersandstone (Ceno- 

 manian) of Koschiitz, Neiderschona, and Goldehen Hohe (Saxom 1. All the 

 specimens obtained are apparently internal casts, so that comparison is very 

 difficult, but they appear to be relatively longer and more obliq le than G. glabra, 

 Sowerby, and the umbo is sharper than in wax casts taken Prom I'.lackdown 

 specimens. The same remarks apply to the Bohemian form referred by Reuss to 

 this species. 



Cucull&a (Idonearca) Shumardi, Meek and Hayden, 1 from the r'-\ Hill Group 

 of Upper Missouri, appears to be very closely related to G. glabra, but without 

 seeing specimens I am unable to state whether it is really distind or no! . 



Remark*. — This is one of the commonest fossils in the Black d >wn < '• reensand. 

 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 tin 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 (PI. 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 (PI. XII, figs, o, 4); (ii) in which the shell is of more 

 rounded outline, more inflated, and has only an indistinct carina 1 I 'I. X [, 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. — I have not seen the type, but the specimens figured b\ Sowerby as 

 G. glabra, G. fibrosa, and G. carinata (all from Blackdown) are in the British 

 Museum. 



Distribution, — Blackdown Greensand (especially zone x). Ganlt ol Folkestone. 



1 • Proc. Acad. Nat. Sci. Piiilad.,' vol 

 U. Missouri' CKep. U.S. Greol. Surv. 

 nl. xxix, fig. I. 



viii (1856), p. 86; Meek, 'Invert. 

 Ten-it.,' vol. ix, 1S76), p. 86, pi. 



t. .hi.: Tert. Foss. 

 Mil. ii-?. 15 a— g\ 



