THECIDEA. 15 



area of attached valve triangular, more or less lengthened, with distinct deltidium ; small 

 valve operculiform, convex, with slight depression in centre, punctuated; the interior of 

 attached valve only known. Length 1 line, breadth the same. 



Obs. I believe this to be the same species as M. D'Orbigny mentions, but which 

 he neither describes nor figures in his Prodrome, under the name of Th. triangularis, as 

 occurring in the Oolitic beds of Ranville, in Normandy, and which name I will here adopt, 

 as, on comparing it with our English specimens, I could find no difference. 



In England this species appears to have first appeared in the middle lias, as Mr. 

 Moore found it attached to Rh. serrata, and it was afterwards found by Dr. Wright, 

 Mr. Morris, and myself, in the Inferior Oolite of the Cotswold and Leckhampton hills, 

 attached to Ter. plicata, Rh. Wrightii, to corals, and to probably any other shell in that 

 bed. In Normandy it is found a little higher up. 



This species also strongly resembles (except in size) the recent Thecidea Mediterraneum, 

 as may be easily perceived on looking at the figure of that species, (Plate I, fig. 13,) which 

 I have purposely placed by the side of Thecidea triangularis. Our fossil species does not 

 appear, however, to have ever attained the dimensions of the recent species. 



Plate I, fig. 11, natural size of Mr. Moore's specimen from the marlstone; fig. 11, 

 a, b, enlarged view of the same. Plate I, fig. 12, natural size of one of the Inferior Oolite 

 specimens; 12«, enlarged. Plate I, fig. 13, recent Thecidea Mediterraneum. 



11. Thecidea Rustica, Moore, MS. Plate I, fig. 4. 



Diagnosis. Of this small Thecidea we are acquainted with only the upper or 

 unattached valve, and therefore the description must consequently be very incomplete. 

 Unattached valve slightly convex, of a squarish circular form, as long as wide, smooth and 

 punctuated, interior presenting two sockets, in which the teeth of the unattached valve 

 articulate from under an elevated crest or lamella, surrounding the shell at some distance 

 from the edge, and, on reaching the frontal margin, it takes a curve towards the middle 

 of the valve ; returning again to the margin, it terminates under the other socket, the 

 position between this ridge and the edge of the shell being strongly granulated, and 

 presenting another smaller ridge, which also joins the sockets, after having gone 

 round the shell. The sinuses observable in Thecideas, and which form so many 

 elevations in its centre, are hardly perceptible in this species ; there is a slight elevation, 

 strongly granulated, on each side of the first described ridge. Length 1 line, breadth the 

 same. 



Obs. The internal organisation of this species is more simple than that generally seen 

 in Thecidea, but it must also be observed that the number of the lamellae, or ridges, forming 

 the sinuses in this genus, and which represent the calcareous supports in Terebratula, vary 

 very much, as may easily be perceived, in casting a glance over Thecidea hippocrepis, tetra- 



