CCELOPLEURUS. 23 



This genus is intermediate between Echinus and Diadema. The known species 

 are all fossil, and either from the upper part of the Cretaceous strata, or from Eocene 

 beds. 



1. Echinopsis Edwardsi. Plate III, fig. 2. 



One of the most interesting of the Echinoderms from British Eocene strata is this, 

 obtained by Mr. Edwards at Brack elsham, and also in the uppermost marine beds at Barton. 

 Two specimens have been found, one showing the upper surface and plates in great perfec- 

 tion, and the other much less perfect, displaying, nevertheless, the general shape, although 

 much crushed. The interambulacral spaces are to the ambulacrals as 3 to 1 . Each inter- 

 ambulacral row of plates is composed of about twelve in longitudinal series Each plate 

 bears a single perforated primary tubercle on the summit of a gently prominent smooth 

 boss. On its inner side is a rather prominent secondary tubercle, and about its base a 

 somewhat irregular circle of granules. The general arrangement of the primary tubercles 

 on the interambulacral spaces is such as to make them appear as two close rows set rather 

 widely apart, thereby contrasting with the closely placed rows of primaries in the ambulacral 

 segments, where, in like manner, there is a single tubercle on each plate. The avenues 

 are broad in consequence of the very oblique transverse series of pairs of pores, falling 

 regularly into ranks of threes, an arrangement noticed as occurring in this genus only 

 in the Echinopsis Gacheti of Desmoulins, from the tertiary of Blaye, a large species as yet 

 undescribed and unfigured. The disk of our Echinopsis is destroyed and the mouth obscured. 

 Associated with it are very slender spines, squamose, with circles of appressed spinules, 

 and in one instance spatulate at the tip. These, judging from their analogy with the 

 spines of the allied genus Astropyga, probably belonged to our urchin. 



Echinopsis Edwardsi measures half an inch in breadth by three tenths of an inch in 

 height. 



Genus — Ccelopleurus, Agassiz. 



Body a depressed spheroid, of five ambulacral and five interambulacral segments, 

 all spinigerous, below and laterally, but becoming naked towards their summits, where 

 they converge to a disk of five perforated genital, and as many perforated very large 

 ocular plates, forming a ring around an apical vent. Tubercles imperforate and bosses 

 simple. Mouth very large. Pores disposed in single file throughout. 



The species of this genus, enumerated by Agassiz, are all Eocene fossils, and are 

 remarkable for singularity and beauty. Unfortunately our only English one is found in 

 the condition of pyritous casts, mostly in the London clay of Sheppey, and presents but 

 very imperfect indications of the peculiarities of its organisation. 



