88 THE FOSSIL ECHINOIDEA 



which flares outwards slightly, narrowest and shallowest near the apical system. 

 The sides of the groove are nearly perpendicular in the midst of the petal, and they 

 slope at the distal and become shallow at the proximal end. The interporiferous zone 

 is well marked and broad, but it is not as broad as one of the poriferous zones. These 

 are unequal, the anterior being the longest and curving outwards and slightly backwards 

 to nearly meet the straighter posterior zone, where the fasciole closes the petal. The 

 pairs of pores are small and close together near the ocular plate ; and there are about 

 7 small pairs in the posterior zones, and 10 in the anterior before the considerable 

 enlargement of the pores commences to be seen. The pairs then increase rapidly in 

 size, are placed nearly vertically, are wide apart, large, and are separated by broad 

 costse ornamented with a linear ridge. There are about 26 large and small pairs of 

 pores in the anterior zone. The angle formed by these ambulacra is rather less 

 than 90°. 



The postero-lateral ambulacra are about half the length of the antero-lateral, are 

 much narrower, shallower, and form a much smaller angle. They are rather close, 

 being separated by a keel, are deep, petaliform, rather rounded at the end, and widest at 

 the junction of the first and second thirds of their length. The interporiferous zone is 

 moderately broad, but it has not the breadth of one of the poriferous zones ; and these 

 are on the nearly vertical side of the ambulacra. The pairs of pores are small and 

 close near the ocular plate, and elsewhere are wide apart and large and separated by a 

 ridge on a costa with or without granules. There are about 15 large and small pairs 

 of pores in the poriferous zones. 



The interradial areas have the plates well marked, and they are usually gibbose, 

 so that the vertical lines of suturing are somewhat depressed. The anterior rise as 

 sharp keels on either side of the odd ambulacrum, and they become narrow and 

 suddenly slope backwards and downwards in front of the line of the apical system. On 

 either side, externally, the keel slopes suddenly to the margin of the antero-lateral 

 ambulacra ; and in front the keels become wider apart, broader, and lower where the 

 fasciole crosses. Beyond that spot the test swells out on either side, being faintly 

 notched along the median line. The lateral interradials are narrowed and broadly 

 keeled near the apex and are tumid. along the plates, where there are globosities. 



There is a sharp, convex, rather broad keel between the posterior ambulacra, which 

 broadens out and becomes lower posteriorly, and it merges into an oval surface more or 

 less truncated, on the upper part of which is a large ovoid periproct. 



The peripetalous fasciole is broad at the terminations of the antero-lateral 

 ambulacra ; it passes backwards and inwards to a nodule on a plate about halfway 

 between the end of the ambulacra and the most projecting part of the keel of the 

 lateral interradium near the apical system. This part is narrow, and becomes small 

 near the nodule. A small portion of test is between the fasciole and the posterior 

 poriferous zone. Then the fasciole, diminished in size, reaches the projection on the 

 keel, and thus is well within the space between the petals. Still narrowing, it reaches 

 the edge of the posterior ambulacra at their outer third, passes round the end of the 



