300 THE FOSSIL ECHINOIDEA 



The radials are large, and extend adorally not very far from a circle which might 

 limit the angles of the basals. The radials are broader than high and are pentagonal 

 in shape. The aboral lateral edges are slightly curved, v?ith the convexity in contact 

 with the concavity of the basal on either side. The adoral and lateral sides are short 

 and inclined downwards and inwards, and are not in curves, whilst the lower or adoral 

 edge is broad and arched, with the concavity towards the ambulacrum. This adoral 

 edge is marked centrally with a projection that reaches a little adorally, and is in the 

 median line of the commencing ambulacrum. This projection covers the position of 

 the exit of the optic nerve, as is usual in the Coelopleuri, and as the plate is not 

 superficially perforated there is reason to believe that the nerve was divided, as in 

 the recent Arbacia, according to Loven. The projection is continuous with a ridge 

 made up of more or less coalesced small granules, which divides the surface of the 

 radial from the apical angle of it downwards. Several more or less distinct ridges 

 and intermediate grooves pass from this ridge obliquely outwards and apically, and 

 others are parallel with the free sides of the plate, and reach the concave adoral edge. 

 This oblique ornamentation is more or less continuous with that of the basal plates 

 in direction. There are usually two of these oblique ridges very well seen ; but there 

 is some variation on different plates in the strength of the ornamentation, and even the 

 central ridge may be absent. The adoral third at least of the free edge of the radial 

 plate is in relation with the apical end of the ridge of granules and very small tubercles 

 which bounds the ambulacrum, and ornaments the outer third of every interradial plate 

 towards the apex. 



The ambulacra are not so broad as the interradia. Above the area of the great 

 tubercles the interradia are only very slightly depressed below the level of the ambu- 

 lacra, and these are very slightly tumid. It has been already noticed that the angle 

 formed by the poriferous zones at the radial end is narrow, and in fact the interpo- 

 riferous area is not broader than the poriferous zone at the level of the eighth pair of 

 pores. In ambulacrum No. III. the breadth does not increase decidedly until the 

 tenth or eleventh poriferous plates, and they are the first of triplets on which a large 

 tubercle is placed. The breadth is greatest where the tubercles are the largest, and it 

 diminishes towards the peristome. Here the breadth is nearly equal to that at the 

 tenth pair on the abactinal surface, and the margin of the ambulacrum is a double 

 adoral curve between the small and broad branchial cuts. This part projects into the 

 peristomial space more than the interradia, and there is a concavity at the median line 

 of the ambulacrum, the edge of which is separated from a large sphferidium pit by a very 

 slender horizontal process, which is between the last of the small tubercles. There are 

 seven large tubercles in one ambulacral zone and eight in the other, and of these three 

 are very large and projecting. The rest are smaller, and diminish in size to the 

 peristome. But above the upper large tubercle there is scarcely any development of 

 tubercle, for although the plates are arranged as in the species in which there are well- 

 developed tubercles in the abactinal part of the ambulacrum, the tubercles are either 

 ill developed or are rudimentary. 



