Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/84

54 The periproct is large and high up. The height of the vertex is about of the length of the test. On either side of the vertex the test slopes down in a bold curve to the ambitus. The apical system is eccentric and in advance of the vertex, and is slightly depressed (Pl. III. fig. 15). There are four large genital openings: the two anterior are closer together than the two posterior, which are separated by the madreporiform body, which extends beyond the posterior; they are large, and each is more or less perfectly surrounded by a circle of miliary tubercles. The ocular pores are large, and form the angles of a pentagon which incloses the generative tract. There is no groove for the odd anterior ambulacrum, which spreads out towards the ambitus, being bounded laterally by a faint ridge on either side, and crossed by the peripetalous fasciole at about of the whole distance from the ambitus to the apical system. There are a few pores crowded near the apex of the ambulacrum, and a few widely apart between it and the fasciole.

The anterior pair of petals are widely divergent, lanceolate, slightly pointed externally; and their poriferous zones are sunken and broad, the hinder ones being the broadest. The pores are rudimentary in the upper part of the front zones; and elsewhere they are largest in the hinder zones. The pores are nearly round, the external row being more or less ovoid; they are conjugate, and the ridges intervening between the pairs are raised and ornamented with miliaries. There are from 24 to 26 pairs of pores in the posterior poriferous zones of these ambulacra. The interporiferous space is slightly broader than the posterior poriferous zone, is convex, and ornamented with large miliaries. The posterior pair of petals, rather close to each other, are slightly shorter than the anterior; they are broader, however, and less pointed at the end. The poriferous zones of these posterior ambulacra are sunken, and are nearly equal in breadth; there are the same number of pores as in the anterior; and the other details are much the same in both. The interporiferous zone is broader than in the anterior pair of ambulacra, and is convex and rather above the ordinary level of the test.

The peripetalous fasciole (PI. III. fig. 15) passes round the extremities of the lateral and of the posterior petals, and across the odd anterior ambulacrum; it passes behind the posterior petals in an irregular course, but on the whole it is elliptical in its outline in the posterior half, and rather angular and wavy in the anterior. It is situated nearer to the apical system than to the ambitus, except anteriorly, where it is nearest to the ambitus. The tuberculation of the abactinal surface below the fasciole is uniform, and the tubercles are smaller near the fasciole and larger towards the ambitus; every- where they are crowded, and in many places there are vacant spots like crescent fascioles. The small secondary tubercles in this part are perforate and crenulate, and the scrobicule is often imperfect and oblique, whilst there may not be a perfect miliary scrobicular circle. Above the fasciole and in the posterior interambulacral space the tuberculation is small, and like that of the interporiferous zones, there being neither large secondary nor primary tubercles. In the