180 THE FOSSIL ECHINOIDEA 



having the apical summit more pronounced, the flatness of the abactinal surface gives 

 place to a gentle declivity on either side of the apical prominence. Seen in transverse 

 profile the outline is a depressed oval, more or less uniformly tumid. The actinal 

 surface is more or less convex, with a low ridge of greatest convexity corresponding 

 with the longitudinal axis of the test, which is, however, scarcely apparent in front of 

 the peristome. The margins are thick, giving the idea of deep sides, and are mode- 

 rately tumid. The passage from the lateral on to the actinal surface is well-rounded 

 and uninterrupted, excepting beneath the posterior truncation, where the rounding is 

 much sharper and subangular. 



The apical disk is excentric in front, its distance from the anterior extremity of 

 the test being proportional to the longitudinal axis as 0*37 : 1. The structure of the 

 disk is compact, with four generative pores, the anterior pair more closely placed than 

 the posterior pair, and the central portion slightly elevated and covered with the 

 punctures of the madreporiform body. 



The ambulacral petals are short and unequal, the odd anterior ambulacrum being 

 slightly the shortest and narrowest, and the posterior pair the longest. The outline 

 of the petals is lancet-shaped, the poriferous zones diverging rather rapidly from the 

 apex, and attain the greatest width at a point nearer the apex than midway between 

 the extremities of the petals ; from thence the zones converge towards the extremity of 

 the petal, which is nearly closed. The poriferous zones are equal and narrow, increasing 

 very slightly in breadth as they proceed outward, but again contracting slightly for a 

 short distance at the outer extremity. The inner series of pores are round, and the 

 outer series are subequal or slightly larger and show the faintest tendency towards 

 becoming comma-shaped ; the pores of a pair are united by a well-defined conjugating 

 furrow, and the divisional costse form narrow ridges, indefinitely granulated. The 

 interporiferous areas have a faintly tumid appearance in consequence of the poriferous 

 zones being slightly sunken in a shallow groove ; the width of the area at its greatest 

 breadth is about three times the width of a poriferous zone or even rather more, whilst 

 at the outer extremity of the petal it is scarcely more than the width of the poriferous 

 zone. The extrapetalous continuations of the ambulacra are invisible in the orna- 

 mentation of the test until close to the peristome, where a few single pores are discernible. 

 The ornamentation of the interporiferous area is similar to that of the interradial por- 

 tions of the test, and consists of small primary tubercles in sunken scrobicules, closely 

 and equidistantly placed, the intermediate space being less than the diameter of the 

 scrobicules and having its miliary granulation entirely confluent. The character 

 of the ornamentation is very homogeneous throughout the test. On the actinal surface 

 a rather narrow, but usually well-defined, band of pitted structure extends from the 

 peristome up to the posterior extremity of the test, on which no scrobicules and primary 

 tubercles occur. 



The peristome is large and excentric in front, lying beneath the apical disk. It 

 is transversely oval or subdecagonal in outline, having a raised rim around its margin, 

 and the peristomial wall extending vertically upward into the body-cavity. The por- 

 tions of this wall which correspond to the interambulacral areas are uniformly granu- 



