340 THE FOSSIL ECHINOIDEA 



than the middle of the posterior extremity. The actinal surface shows great convexity, 

 especially in the region of the plastron. The sides are thick and tumid. 



The apical disk lies at the base of the sloping interradial keels, and is remarkable 

 for its apparent breadth, caused by the lateral pair of plates being wide apart. There 

 are four genital pores, the posterior pair being larger and slightly wider apart than the 

 anterior ones. They, as well as the posterior pair of ocular plates, are separated by the 

 large wedge-shaped central madreporiform body. The anterior pair of ocular plates 

 appear to have been extremely small, but the condition of the fossil does not show 

 them satisfactorily. The odd anterior ocular plate is small and placed midway between 

 the anterior pair of genital plates. 



The odd anterior ambulacrum lies in a well-defined, tolerably deep, and broad 

 groove, the margins of which are sloping, and the lateral keels are not remarkably high 

 or thin, although they become angular and well defined as they approach the apical 

 disk, and at the same time slope away. The floor of the groove is slightly rounded, 

 and the poriferous zones are placed at its junction with the sloping lateral walls, 

 appearing rather more on the floor than otherwise, at least near the apex. The pores 

 are small and slightly elongate transversely, and are separated by a small granuliform 

 prominence. The plates are very short and band-like and successive pores are con- 

 sequently closely placed. The plates are further remarkable for the abruptness of the 

 bending which occurs just internal to the inner pore of the pair, the portion of the 

 plate which lies in the interporiferous area being reflected sharply backwards towards 

 the apex at about 30° from the direction of the outer part of the plate (see fig. 9). It 

 will also be noticed that the facets of the inner end of the plates have a peculiar 

 curvature. Traces are visible of a few widely spaced granules on each plate, ornamenting 

 the interporiferous area. 



The anterior pair of ambulacral petals are large, wide, petaloid, placed in tolerably 

 deep grooves, and are rather divergent, the included angle being, roughly speaking, 

 about 90°. The grooves, which are wide and gracefully rounded outwardly, contract 

 very little in breadth until they reach the proximal third of the length, where they 

 commence to taper rapidly to a point. The anterior poriferous zone is bent with a slightly 

 sigmoid curvature. The companion posterior zone proceeds for some distance from the 

 apex at a right angle to the longitudinal axis of the test, is then rather sharply bent with 

 a well-rounded curve, and proceeds almost straight towards the margin. It will thus be 

 seen that the bending of the petal on its approach to the apex is very conspicuous, and 

 that there is also a slight inflation of the petal near the distal extremity. The pori- 

 ferous zones are wide and fully twice the breadth of the interporiferous area about the 

 middle of the petal. The pores are elongate except near the apex, rather wide apart, 

 distinctly conjugate ; with adjacent pairs separated by a well-defined costa, which was 

 probably ornamented by a row of granules. The first 10 or 12 pairs of pores in the 

 anterior poriferous zone of the antero-lateral petals, and the first 6 or 7 in the posterior 

 zone, are remarkably small, and those beyond increase in size very abruptly. The 

 posterior pair of petals are about two thirds the length of the anterior pair, lanceolate 

 in outline, with the greatest breadth a little removed from the middle of the petal 



