BRITISH FOSSILS. 



3 



them, and consequently as iDany ambulacral plates, correspond to each 

 interambulacral. On the ventrical surface they become doubled in 

 number, so as to form rows of twos, and near the mouth, where the 

 ambulacral plates widen considerably, two such rows go to each plate. 

 The primary tubercles are all perforate, and the rim of the protuber- 

 ance in which they are set is crenulated. 



The mouth is small, obscurely pentagonal, and in diameter equal to 

 one-fourth of the distance between it and the margin. The anus is 

 oblong, and somewhat acute at each extremity. It occupies rather 

 more than one-fourth of the space between the mouth and the margin, 

 and is distant from the margin its own longitudinal diameter. It is in 

 the centre of an inferior interambulacral space, and is as if cut out of 

 the plates, its margins being on a level with them, except the inner 

 extremity, where there is a bulging. 



The apex is composed of the combined ovarian and ocular plates, 

 and is often slightly prominent, in consequence of the convexity of the 

 ovarian plates, both individually and collectively. The centre assem- 

 blage forms a slightly oblong or obscurely pentagonal area, conspicu- 

 ously divided into five parts by the five ovarians. Of these the left 

 antero-lateral is largest, and appears to represent a madreporiform 

 plate, and the left postero-lateral smallest. The other three are of 

 about equal dimensions. The ovarian pores are placed near the outer 

 and central edges of each plate, except the posterior one, which is 

 obsolete. The ocular plates are very small, obscure, and triangular. 

 The ocular pores are developed in all five. The greatest diameter of 

 the apical crown of plates is less than the breadth of the mouth. 



Localities and Geological Position. — In the chalk marl and lower 

 chalk, Hamsey, near Guildford (Mantell). Markham Gayton (Wood- 

 ward). Charing (Morris). Lewes, Dover, Burham, near Maidstone 

 (Survey Collections). 



Foreign Distribution, — In many localities (chalk marl) of France 

 and Germany. 



Description of the Plate. 



Fig. 1. Hemispherical, or half-grown form of the species (Survey Collection) seen in 

 profile, and surrounded by an outline representing the perfect and cylindrical contour of a 

 full-grown specimen (in the British Museum). Figs. 2 and 3. The former specimen seen 

 fiom above and below. Fig. 4. Diagram of the mouth ; and Fig. 5, of the anus. Fig. 6. 

 Centro-lateral, ambulacral, and interambulacral plates, with their tubercles, granules, and 

 pores. Fig. 7. Ambulacral and interambulacral plates, &c., of ventral surface. Fig. 8. 

 A primary tubercle, with surrounding granules, from the ventral plates. Fig. 9. A 

 similar tubercle, from a dorsal plate. 



