22 ECHINODERMS OF THE LONDON CLAY. 



1. Cidaris Websteriana. Plate III, fig. 4. 



I have given this name to the spines of a Cidaris collected by Mr. Edwards at 

 Barton, the only relic of the genus as yet noticed in British tertiaries. Their shape 

 is stout and cylindrical, slightly swelling out at their halves, ornamented by about twelve 

 longitudinal rows of rounded and nearly equal tubercles, the grooves between being very 

 narrow and very minutely striated. Between the collar of the spine and the com- 

 mencement of the ridges of tubercles, is a shallow well-defined nearly smooth neck. The 

 collar is not crenulated ; from within it projects the nipple-like deeply hollowed articular 

 surface. A large example measures -f-2-ths of an inch in length by -^th in diameter at its 

 most tumid part. 



In the Museum of Practical Geology ; presented by F. E. Edwards, Esq. 



Family. — Echinid^e. 



Genus — Echinus, Linneeus. 

 (See page 2.) 



1. Echinus Dixonianus. Plate III, fig. 3. 



As yet no body, or even plate, of any true Echinus has been found in British 

 Eocene strata, A single spine belonging to this genus has, however, been procured by 

 Mr. Edwards at Barton. It evidently formed part of the armature of a large sea- 

 urchin. It is ornamented with about twenty-four depressed, rather narrow longitudinal 

 ribs, separated from each other by very fine grooves, and at their lower parts as if 

 doubled, owing to the presence of a finer groove down the centre of each rib. The collar 

 of the spine is raised, tumid, and crossed by the grooves. The neck is pyramidal and 

 truncated, with a rather small articular surface. The spine itself is imperfect; but when 

 entire, may have measured three quarters of an inch in length by one eighth in breadth at 

 the collar. 



Genus — Echinopsis, Agassiz. 



Body spheroidal, inflated, with homogeneous ambulacra converging above to a genital 

 disk, composed of five perforated genital plates alternating with five perforated oculars, 

 and forming a ring around an apical vent. Mouth central and inferior. Tubercles 

 perforated, as in Diadema, but borne upon plain and not crenulated bosses. 



