120 * THE FOSSIL ECHINOIDEA 



thus separates the adjoining pairs of pores. The inner pore of each pair is close to the 

 adoral margin of the plate, often excavating it ; the outer pore is midway between the 

 margins. There are three poriferous "plates to each whole ambulacial plate. The inter- 

 poriferous areas have two rows of primary tubercles, about 18 to 20 in each, one 

 standing close to each poriferous zone. . The tubercles are crenulate, have a moderately 

 large imperforate mamelon, and are little, if at all, less than the primary interambu- 

 lacral tubercles. A few irregularly-disposed and widely-spaced granules occupy the 

 plate on the^ inner side of each primary tubercle. Near the ambitus, and extending up 

 to the peristome, one of the granules in each plate is much larger than the rest and is 

 mammillated ; and these form two alternating series of small secondary tubercles. Each 

 of the marginal primary tubercles is separated from its neighbour by a regular trans- 

 verse line of three small granules, which stands on the aboral margin of the plate. 



Interambulacral areas wide, twice and a half the breadth of the ambulacral 

 areas, and are furnished with two rows of primary tubercles, 15 to 17 in each ; these 

 are placed rather nearer to the poriferous zone than midway on their respective plates. 

 At the ambitus there is a moderate-sized secondary tubercle at either side of the pri- 

 mary. The inner series does not extend very far abactinally above the ambitus, and 

 becomes insignificant towards the peristome. The outer series extends rather further 

 on the abactinal surface, but disappears entirely about midway between the ambitus 

 and apex. The primary tubercle is surrounded by a more or less distinct circle of small 

 miliary granules, which is confluent with the neighbouring circle on the lower margin. 

 The plate is further ornamented with a small number of widely-spaced, irregularly- 

 disposed, unequal granules, and a well-defined, naked, miliary zone is preserved from 

 the ambitus up to the apical disk. The primary tubercles maintain their size very 

 uniformly throughout the series, the diminution towards the extremities being very 

 slight. There are eight or nine pairs of pores opposite two of the interambulacral 

 plates of the ambitus. 



Peristome decagonal, small, impressed ; diameter rather more than one third the 

 diameter of the test. Mouth-slits rather deep, wide, and well rounded, margined with a 

 slight rim. 



Apical disk well developed, though forming a rather narrow ring ; periproctal aper- 

 ture suboval, with the right posterior ocular plate always iticluded. The other oculars 

 are smaller, regularly pentagonal in outline, and the puncture is minute and situated at 

 the extreme edge of the adoral margin. The genital plates are somewhat broader than 

 long, the adoral prolongation rather acute ; the genital pores are large and oval, and 

 placed nearer to this angle than to the periproctal margin. The right anterior genital 

 plate is scarcely larger than the others, and bears a compact prominent subtubercular 

 madreporiform body ; the other four genitals have a row of four or five granules along 

 their margin, and there are two or three on the included ocular. The excluded oculars 

 have a single granule on the centre of the plate. 



Variations. The form, as a whole, is remarkably constant, the chief variations 

 noticed being those which afi"ect the marginal contour, tending to a more or less penta- 

 gonal outline, in consequence of the inflation of the radial areas, and the greater or less 



