FROM THE WHITE CHALK. 75 



anotliei". A transverse section of spines so affected proves that the canals are of neither 

 uniform length nor dimensions, some being of greater extent and more open than 

 others . 



" The central perforation not unusual at the apex of some spines, seems also due to 

 disease or to parasitic borings, and will often be found to extend downvi^ards as far as 

 the acetabulum. This is the case with the spine drawn in PI. V, fig. 6. A portion of the 

 surface of this spine having been cai'efully removed subsequent to the drawing being made on 

 the plate, the cavity apparent at the apex was seen to extend through the whole length of 

 the body, unaltered in size ; just below this point it suddenly contracted in a circular 

 curve (similar to the base of the perforations made by a Pholas) as though to avoid break- 

 ing through the walls of the neck ; at the lower point of the circular excavation the opening 

 appeared again, only with a very much less diameter, and extended as far as the articular 

 cavity, through which it passed. Another spine open at the apex, when cut lengthways, 

 gave the same result (of a continuous tube, of two different diameters), except that the 

 opening, which extended almost as far as the acetabulum, did not pierce it, but passed 

 outwards in a transverse direction. 



" The test of C. davigera varies in the proportions of its parts from youth 

 to age ; my smallest example, five tenths of an inch in width, differs consider- 

 ably in appearance from my largest, which is one inch and seven tenths in 

 width. Comparing these two it is seen that the tubercles in the former are 

 relatively larger than in the latter ; that the granules of the miliary zone are in 

 the former almost as large as in the latter ; that the number of plates are the same 

 in both ; that there is an oval rudimentary tubercle in the uppermost plate of the 

 anal side in the largest specimen ; that the areolas of the two superior tubercles 

 of the anal side are in the smallest example separated by only three granules, including 

 those of the scrobicular margin, whilst in the largest example there are fourteen. Both 

 specimens have four rows of granules in the ambulacral areas at the ambitus ; in the 

 smallest they are of equal size and equally disposed, in the largest the two interior are 

 much smaller than the two exterior, more numerous, and crowded together. These dif- 

 ferences have a tendency to cause the two specimens, when placed with the anal side 

 uppermost, to appear very dissimilar, particularly in the region of the miliary zone. 

 Of these two specimens the smallest is much below and the largest much above the average 

 size. 



" Spines of C. davic/era are sometimes, but very rarely, found as far down as the 

 middle of the flinty Chalk ; the proper horizon of C. davigera is above this part. 



" Figures 1 a, 1 (5, 2, 3 a, 3 (5, 3 <?, 4 a, ^h,^ a,h b, PL XHI, are from the Upper 

 Chalk of Bromley, in Kent. 



