661 
degree flattened ; umbo sharp and projecting ; dental plates, as indicated by the 
impressions left, strong, short, and rather curved ; flanks bearing six prominent angular 
ribs, and the sulcus occupied by four fainter costae, with indistinct traces of distant 
transverse lamellae. Dorsal valve depressed, with a faint flat fold ; septum very short ; 
costa) similar to those of the ventral valve. 
Ols. This Ttliynohonella is common in the Croydon G-rit, but always in the form 
of easts, and those usually of the ventral valve. At first sight it would appear to have 
some resemblance to H. lacunosa, Sehl., from the White Jura of G-ermany, although the 
outline is decidedly more that of R. coinpressa, Sby., of the European Cretaceous, but 
the costas in our shell are too few and coarse for that species. With the exception of 
the inequilateral feature in Rhynohonella mteriana, D’Orb., our shell comes very close 
to this. The specimens are not large for a Jurassic Rynchonella, but they are so as 
compared with the generality of the Cretaceous forms, and when compared with 
Tertiary or Recent species are very large. 
Another shell allied to R. croydonensis is R. major, J. de C. Sby., sp.,* from the 
TJpj)er Mesozoic Beds of Cutch, but the costa? are far too numerous and close. 
Associated with R. croydonensis is a smaller cast, apparently that of a dorsal 
valve. It possesses a much more regular degree of convexity, and is without a mesial 
fold, but there is present the same flattened umbonal region as in the above. Its more 
convex form generally and the want of a fold forbids it being the dorsal v.alve of R. 
croydonensis, but at the same time the two forms do bear an indescribable resemblance 
to one another. As in the former case, the septum is short (PL 42, fig. 8). 
Loc. True Blue Hill, Croydon Gold Pield (R. L. Jach) ; Mount Angus, Croydon 
(R. L. JacTc). 
Section-MOLLUSCA VERA. 
Class — Pelecypoda. 
Oider-OSTRACEA, 
Eamily— OSTEEID^. 
Qenus — OSTRRA, Linnems, 1758. 
(Syst. Nat., Ed. x., p. GOO.) 
OsTREA, sp. ind., PI. 43, fig. 8. 
Ohs. Some obscure impressions and easts from the Croydon Grit appear 
referable to this protean and widely spread genus. It is difficult to assign any definite 
characters to them, but the casts are irregularly sub-obovale, oblique to some extent, not 
greatly widening towards the front. The umbo of the under valve is thickened although 
not curved upwards, but the left-hand edge or margin of the shell deeply sinuated. The 
grooved cartilage area is roughly triangular, and obliquely inclined towards the sinuated 
side. 
This resembles many forms of Oyster, but is not unlike a variety of Ostrea 
patina, which the late Mr. Meek proposed to call Ostrea suhsinuata,j except that the 
sinuated margin is on the right side of the attached valve ; in our specimens it seems to 
ho on the left. In addition to this the ligameutal depression is oblique, but in Meek’s 
figure it is not so represented. 
There is not sufficient evidence to warrant a specific name being given to these 
specimens. 
Loc. True Blue Hill, Croydon Gold Eield {W. SamwclT). 
* Trans. Geol. Soc., 2nd Ser., v., E,-!!)!. t. 22, t. 22, f. IG. 
t Report U. S. Geol. Survey Ter., 1876, ix. (Invert. Oret. Tert. Eosa. Up. Missouri), p. 18, t. 11, 
f. 4a and 46. 
2 M 
