FROM THE WHITE CHALK. 71 



of granules in the ambulacral areas at the ambitus {C. sceptriferam. specimens of the same 

 size as that under consideration having eight at the equator, whilst this species has six), — 

 in these granules being more irregularly disposed and more crowded together, — in the 

 proximal discal plate being marked with a more prominent tubercle, and in its shape 

 being less elongated, — in the areolas being relatively larger, — in the spines being uniformly 

 cylindrical instead of fusiform, and of much less diameter, — and in the serrated ridges of the 

 spines being fewer, wider apart, and continuous the whole length, whilst in C. scepfri/era 

 some of the ridges cease at the widest part of the spine. C. intermedia differs from 

 C. subvesiculosa in the scrobicular margins of adjacent plates being less widely separate, 

 — in the granules on the margins of the areolas being more distant, — in the sutures of 

 the miliary zones being less marked, — in the miliary zones being smaller, — in the spines 

 being of less diameter, with less numerous ridges, and apparently shorter (some 

 spines of C. subvesiculosa, of a test of equal dimensions, reaching a length of three inches), 

 — and in the plates presenting a flatter and less tumid appearance. 



" Locality/ and Stratigrapldcal Position. — Collected from the White Chalk of Sussex, 

 apparently from the base of the Chalk-with-flints ; rare. The specimen figured, Plate XH, 

 fig. 1 a, is of the natural size. Fig. 1 h, one of smaller spines magnified, length 1 inch, 

 diameter y'^th of an inch. 



"Additional Notes on CIDABIS CLAVIGURA, Konig. (See p. 48.) 



" Very marked as are the variations in the general aspect of the spines of C. claviyera, 

 it will usually be found that a single and prevailing form is connected with each indivi- 

 dual test. On PI. XHI are drawn the tests and spines (figs.'il a, 3 a, 4 a) of three speci- 

 mens, in which the spines attached to the tests are tolerably uniform in shape in each case 

 collectively, yet arc dissimilar when viewed by groups, those of fig. 1 a being all cla- 

 viform, those of fig. 3 a being all medially constricted, those of fig. 4 a being all fusiform. 

 The same remark holds good in other examples not drawn on the plate. I have now 

 before me sixteen specimens of C. claviyera, with the spines jattached, in all of which speci- 

 mens, although as a general character each company of spines has a club-shaped or approxi- 

 mately club-shaped contour, there is so great a variableness among the different groups 

 that if in any group the two extremes in form were to be compared apart from the test 

 they could easily be mistaken for different and distinct species ; some (No. 1 of the Table 

 on page 72) being wholly cylindrical, these by easy gradations seen in sets of forms passing 

 on so as to become pear-shaped (No. 4), next taking up the ordinary clavigerous type 

 (Nos. 6, 7, S), and ending with those having the medially constricted outline (No. 10). 



" 1 append woodcuts of some of these varieties, giving their dimensions in tenths of 

 an inch, and also the diameter (major axis) of the test to which they belong. The 

 measurements of the spine in each case have been derived from a specimen which in its 

 natural position Avould have been affixed to the ambitus. 



