290 THE FOSSIL ECHINOIDEA 



gradually diminishes, but not to a point ; for, as has been noticed, there is a rather 

 broad peristomial termination. 



The area is crowded with great tubercles, which begin just above the ambitus and 

 reach below it. Above there are a few smaller yet well-developed tubercles, and 

 actinally they diminish to the peristome, where the crowding is greater than it is 

 above. Still smaller primary tubercles occur at regular intervals close up to the radial 

 end, and the first great primary, smaller, however, than the next in succession towards 

 the ambitus, is clearly higher up on the test than the highest great tubercle of the 

 interradia. 



In full-grown individuals there are at least eighteen fully-developed tubercles, and 

 about four or five which are small and imperfect in their arrangement of the triplets. 

 Eight of the perfect tubercles are very large and the rest decrease in size to the peri- 

 stome, but they are unlike the apically situated small tubercles in shape and pore 

 arrangement. In all the mamelon is small, has a constricted neck surrounded by a 

 shallow groove, and a large conical inward and upward sloping boss, which has a wide 

 flat base, which may extend quite to the edge of the plate, or leave a narrow space there 

 at the median line of the zone for a single row of small granules. This row of granules 

 may extend on to the line of suture between the successive plates on either side of the 

 median line. 



The perfect primary tubercles consist, as is usual in the genus, of a large primary 

 plate which carries the mamelon and the greater part of the boss and all of it near the 

 median line of the area ; and of two demi-plates, one placed abactinally, and the other 

 actinally in relation to the central primary of the compound plate. 



These demi-plates form the aboral and the adoral flanks of the boss of the 

 tubercle, but do not enter into the composition of the mamelon. Neither do the 

 sutures of these demi-plates come in contact in any way. As is usual in the genus, the 

 suture or edge of the aboral demi-plate, which is indented by the inner pore of the 

 pair, passes inwards up the side of the boss to reach the groove at the abactinal edge of 

 the neck of the mamelon, and it then passes either directly abactinally, or abactinally 

 and slightly externally (Plate XXXIX, Fig. 12, Nari Series). In the first case the 

 suture will end by joining the adoral sutural line of the plate immediately higher in 

 the test, and in the other the suture will pass so as to reach the adoral pore of the 

 higher plate. The adoral demi-plate is of the same shape as the other demi-plate. Its 

 aboral suture is of course in contact with the adoral edge of the poriferous part of the 

 central or primary plate of the triplet, and after being in relation with the adoral pore 

 of the primary, its line of suture passes up the boss to the groove at the edge of the 

 neck of the mamelon and then turns either directly actinally, or actinally and slightly 

 externally, so as to reach the line of suture between the consecutive great tubercle- 

 bearing plates, or to reach its own adoral pore. 



The shape of the central plate of the triple combination is very remarkable. The 

 poriferous part, situated between the poriferous parts of the demi-plates above and 

 below, extends from the interradial edge inwards and includes the space as far as the 

 edge of the tubercle. The rest of the plate comprises the portion of the tubercle 



