24 ECHINODERMS OF THE LONDON CLAY. 



1. Ccelopleurus Wetherelli. Plate III, fig. 1. 



Achosalenia, species of, Morris. Cat., p. 47. 



Depressed, orbicular, convex above, plain below, with somewhat compressed sides. 

 Ambulacral areas very narrow, smooth above, ornamented at their inferior portions by 

 a few large alternating tubercles. Interambulacral spaces also quite naked above, and 

 similar, bearing few large alternating tubercles in their lower portions. The tubercles of 

 lower parts of the segments occupy the whole of the base around the very large mouth. 

 The avenues are broad, though composed of pores ranged in single file, this arrangement 

 becoming obscure near the mouth. The apical disk is remarkable for the very large 

 ocular plates. In consequence of the bad condition of the tests, both tubercles and pores 

 are often obliterated. In Mr. Wetherell's collection, is a specimen apparently of this 

 species from Highgate Archway, with the spines imperfectly preserved. They were long, 

 slender, and longitudinally grooved, the ridges few and granulated. 



Most of the numerous examples of this urchin have been procured in Sheppey by 

 Mr. Bowerbank. The largest example measures half an inch in diameter by a quarter of 

 an inch in height. 



Family. — Spatangid^e. 

 (See page 13.) 



Genus — Hemiaster, Desor. 



Inflated urchins with heterogeneous ambulacra, distinctly petaloid in their dorsal 

 portions, and often lodged in depressed spaces. They have terminal vents and excentric 

 bilabiate mouths. The petals of the back are circumscribed by a slightly undulated fasciole. 

 There is no sub anal or intrapetal fasciole. There are no large primary tubercles mingled 

 with the very uniform secondaries that cover the plates. All the known species are from 

 Cretaceous or Eocene strata. They are very similar to each other in general aspect, 

 so that it is requisite to have recourse to critical distinctions in defining the species. 



1. Hemiaster Bowerbankii. Plate III, fig. G. 



This little urchin is very tumid, much elevated posteriorly, declining anteriorly. Its 

 highest portion is just above the truncated anal extremity. The contour is obcordate, 

 with an obscure tendency to a hexagonal outline. The dorsal ambulacra are widely 

 petaloid, very unequal, and all lodged in deep excavations. The antero-lateral ones are twice 

 as long as the postero-laterals. The latter are broadly ovate, and have about eight pairs 



