296 HYBOCLYPUS. 



have never seen any of the elements of the disc, I am unable to judge of the value of the 

 character on which the separation of this urchin into a new genus is proposed to be made. 



Htboclypus caudatus, Wright. PI. XXII, fig. 2 a, b, e, d, e,f,g,h, i,j,k. 



Hyboclypus caudatus. Wright, Annals and Magazine of Natural History, 2d series, 



— — vol. ix, p. 100, pi. 3, fig. 2 a — e. 



— — Forbes, in Morris's Catalogue of British Fossils, 2d ed., p. 82. 



— — Desor, Synopsis des Echinides Fossiles, p. 193. 



Test small, oblong, much depressed ; the single inter-ambulacrum produced into a 

 caudal prolongation ; mouth very near the anterior border ; apical disc and vertex excen- 

 tral; anterior border rounded and elevated, the posterior produced, and truncated. 



Dimensions. — Height, nine tenths of an inch ; antero-posterior diameter, one inch and 

 two tenths ; transverse diameter, one inch and one tenth. The great majority of the 

 specimens, however, have the following measurement : Height, seven twentieths of an 

 inch ; antero-posterior diameter, fifteen twentieths of an inch ; transverse diameter, 

 thirteen twentieths of an inch. 



Description. — The test of this elegant little urchin has an oblong shape, rounded and 

 elevated before, produced and truncated posteriorly, and having the mouth and apical disc 

 excentral, placed much nearer the anterior than the posterior border ; the surface of the 

 plates is covered with very small tubercles, which require the aid of a good lens to dis- 

 cover; without this the observer might suppose the test was altogether destitute of 

 sculpture (fig. 2 a, b) ; in consequence of the excentricity of the mouth opening and disc, 

 the single and antero-lateral ambulacral areas are straight and short, and terminate at 

 the anterior border of the disc (fig. 2 a) ; the posterior pair are one seventh longer than 

 the anterior pair ; they curve upwards, inwards, and forwards on the dorsal surface, and 

 terminate by the margin of the longitudinal valley, at a short distance from the posterior 

 border of the disc (fig. 2 a). 



The poriferous zones are narrow; the pores are situated some distance apart on 

 the"dorsal portion of the zones, and much wider apart at their basal region (fig. 2 b,f). 



The inter-ambulacral areas are of unequal width ; the anterior pair are the shortest 

 and narrowest, the posterior pair the widest, and the single area the longest ; it is 

 likewise considerably produced into a lip-like process, which curves gently downwards, 

 and is abruptly truncated posteriorly (fig. 2 a, b) ; the anterior and posterior pairs of the 

 inter-ambulacral, and all the ambulacral areas, are convex on the upper surface ; but the 

 single inter-ambulacrum is traversed superiorly by a deep, broad, longitudinal valley, with 



