160 THE FOSSIL ECHINOIDEA 



The apical disk is small ; and there are four genital pores, which are large and 

 oval, occupying nearly the whole of their plate; the anterior pair rather closer 

 together than the posterior pair. The ocular plates are small and their pores very 

 minute, and the posterior pair are wide apart. The whole of the central portion 

 is occupied by the madreporiform body, and this, in some instances, is slightly convex. 



The ambulacral petals are narrow, and of almost uniform breadth a little distance 

 from the apical disk, expanding very slightly near the middle of their length, and the 

 contraction at the outer extremity being also very insignificant. They terminate some- 

 what abruptly at a short distance before the rounding of the margin commences. The 

 posterior petals are the longest, and the odd anterior petal is slightly narrower than 

 the others. The breadth of one of the paired petals is in the proportion of about -^Sh 

 of the length of the test. The poriferous zones are unequal ; the anterior zones of the 

 antero-lateral petals and the posterior zones of the postero-lateral petals being shorter 

 than the companion zone of the same petal. The breadth of the poriferous zone, 

 excepting the narrow apical extremity, varies only slightly throughout the remaining 

 portion of its length, the increase in size at the widest part, and likewise the decrease 

 towards the outward extremity, being slight. At the widest portion of the petal the 

 width of the poriferous zone barely exceeds one fourth of the width of the inter- 

 poriferous area, the actual breadth of the zone being only a shade more than 1 millim. 

 in an example 58 millim. in length. The inner pores of a zone are round, and the 

 outer pores elongate or pyriform, the pair being united by a well-defined channel. 

 The divisional septum between each pair is rather broader than the conjunctive 

 channel, and is ornamented by a single line of small miliary granules. At about one 

 fourth of the distance from the apical to the outer extremity of the postero-lateral 

 petals the anterior zone is bent slightly outward, appearing to curve somewhat 

 encroachingly upon the postero-lateral interradial area, whilst the posterior zones are 

 comparatively straight and do not correspondingly encroach upon the odd posterior 

 interradium. This gives the character of a slightly outward flexure to the posterior 

 petals. At the extremity of the petals, where the conjugate pores cease, the poriferous 

 zone is continued in a single line of pores, sunken in a faint depression or channel, and 

 extending up to the peristome, in the neighbourhood of which there is some irregularity 

 in the serial arrangement of the pores consequent on the presence of intercalated 

 ambulacral plates, by which the incipient phyllode is produced. The lines formed by 

 the simple portion of the two poriferous zones of a petal expand wider apart as they 

 proceed from the extremity of the petal to the ambitus, and again converge from the 

 ambitus to the peristome. The course of the three anterior ambulacra on the actinal 

 surface is comparatively straight, that of the posterior pair being more or less curved, 

 bending with a slight and graceful flexure which encroaches on the odd portions of 

 the interradial area. The interporiferous areas are flush with the surface of the test, 

 both within and without the petal, and their ornamentation is similar to that of the 

 interradial areas. 



The antero-lateral and the postero-lateral interradial areas present no tumidity 

 either in the abactinal or actinal regions. The odd posterior interradium is slightly 

 tumid dorsally over, and in conformity with, the posterior rostration ; and there is 



