FROM THE GREY CHALK. 77 



are large and elongate on the upper half of the body, pointed at the apical region, 

 circular on the lower half of the body, diminishing in area as they approach the neck, 

 and ceasing at that part, arranged in rows gradually increasing in number from 

 the apex to the greatest diameter, and afterwards more closely and less regularly 

 deposited ; the neck smooth, short, and very much contracted ; the milled ring is slightly 

 prominent, covered with fine longitudinal striae ; the head smooth. 



" The spines of C. Bixoni occur at the base of the Lower or Grey Chalk in the cliffs 

 between Folkestone and Dover, in the band containing the spinous Ostrea carinata, Sow. 

 (M.C., tab. 365, fig. 1), in company with C. Bowerbankii ; they are, however, very rare. 

 The same species is found occasionally in the " Coprolitic Bed " of Cambridge, a deposit 

 containing rolled fossils from the Lower Chalk, Upper Greensand, and Gault formations. 

 The specimen figured in PI. XH, fig. 6, and obtained from the Coprolitic Bed of 

 Cambridge, is identical in all respects with the Folkestone examples, except that the 

 surface is more worn, and appears to have been subjected to much friction ; the width of 

 the Cambridge specimen is seven tenths of an inch, length of body one inch. The 

 total length of spine (measured from a specimen in perfect condition in my cabinet), 

 from Folkstone is one inch and four tenths ; greatest diameter (midway between apex 

 and edge of acetabulum) seven tenths ; length of head and neck three twentieths ; 

 diameter of neck three twentieths. 



" The test of this Cidaris has not at present been discovered ; it would appear, however, 

 judging from the analogy of its spines with those of C. Bowerbankii, that it must have 

 much in common with the latter, except size ; perhaps it may be an aged form of 

 C. Bowerbankii. 



" Additional Note on CIDABIS BOJFEBBANKII, Forbes. (See p. 45.) 



" In this species, as in others of the Cidares, the form of the spine varies according to 

 its position on the test ; those at the peristome are tolerably cylindrical, with an acute 

 apex ; those at the ambitus are inversely conical, with the apex less acute, and those at 

 the anal margin have the body inflated and the apex somewhat obtuse. The peri- 

 stome spines have the surface covered with coarser granulations than is the case with 

 those which occur on the opposite side. At Southeram Pit, near Lewes, Sussex (Lower 

 Chalk), tests with the spines in situ are occasionally found. In my cabinet is a specimen 

 from Southeram Pit nearly perfect, in which almost the whole series of spines, from the 

 anal to the oral region, are in position, and in which the variation of form in these spines, 

 according to their situation, is well exhibited. From this specimen were drawn the figures 

 shown in the woodcuts fig. 4 a — d, which are twice the size of the originals ; a is the spine 



