60 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



specimen of R. Mantelli in the British Museum, small Oysters were 

 attached to the interior at least 10 inches down the cavity. I find, 

 however, that in Dr. Mantell's specimens a portion of the thin na- 

 creous layer of the Radiolite is preserved beneath these parasitic 

 bivalves. 



A very instructive specimen of R. Mortoni has lately been ob- 

 tained by Mr. Matthew Wright from the Upper Chalk of Kent* 

 (PL V. fig. 1). It consists of the upper portion of a large cylindrical 

 example, measuring 6 inches across ; the cellular tissue is empty, 

 except in two places where nodules of flint seem to grow from it ; 

 and the rim is broken, and incrusted with Oysters. The cavity is 

 oval, and occupied at the lower end by a mould of indurated chalk, 

 rather smaller than the cavity ; the space between, from \ line to 

 3 lines, was filled with very soft ferruginous chalk, representing the 

 inner layer of shell. On one side of the mould are two pairs of 

 longitudinal furrows, indicating projections from the shell- wall, which 

 originally received the dental processes of the upper valve. Part of 

 this mould has been detached, and a reduced side-view of it is given 

 in PI. V. fig. 2. It is divided into joints by partings of soft chalk, 

 replacing the septa of the water-chambers. These septa, as well as 

 the outer wall, had been perforated by Clionce. 



The upper end of Mr. Wright's fossil was filled with soft white 

 chalk, on removing which with great care several Oysters and Spon- 

 dyli were found to have grown inside, in places where the nacreous 

 layer was less than half a line in thickness. One of the Spondyli 

 bends outwards just at the point where the anterior dental socket 

 is still indicated. The dental sockets are grooved, as in R. mammil- 

 laris (fig. 10), but are situated close to the shell-wall, and at some 

 distance apart, as in R. calceoloides. This species, R. Mortoni, is 

 most nearly allied to R. cornu-pastorisy Desm., the type of D'Or- 

 bigny's genus Bi-radioliteSy a section the value of which is not yet 

 known. 



6. Radiolites Mantelli, n. sp. PI. V. fig. 4. 



Shell elongated, grouped, furrowed and striated lengthwise ; furrows 

 large, rounded, divided by acute ribs ; rim steeply inclined, radiately 

 striated, cellular near the inner margin ; shell-wall thick, finely 

 laminated, the laminae divided vertically by very close radiating 

 plates, passing into minute cells near the interior ; interior smooth, 

 marked by lines of growth and a narrow ligamental furrow : inner 

 layer wanting. Upper valve unknown. 



Length of fragment 10 inches. 



This species occurs in the Upper Greensand of Cap la Heve, near 

 Havre ; it is very distinct from any species described by M. D'Or- 

 bigny, and being likely to occur in England, I have noticed it here, 

 and named it after the distinguished geologist who first discovered 

 Radiolites in this country. 



* Mr. M. Wright has obUgingly informed me that this specimen was found in 

 the Upper Chalk in the quarry to the west of and contiguous to the Rosherville 

 Gardens, in a horizontal layer of flints, at about 30 feet from the surface. 



