FROM THE INFERIOR OOLITE. 297 



vertical and parallel walls in the upper part, gradually expanding into two ridges at the 

 lower half, and which form the lateral boundaries of the anal valley (fig. 2 a). 



The anterior border is bluntly rounded (fig. 2/), with a slight depression in the middle, 

 formed by the single ambulacral area ; the base is concave, and slightly undulated 

 (fig. 2 c), in consequence of the basal portions of the inter-ambulacral areas being convex, 

 and those of the ambulacral forming straight valleys between them (fig. 2 I). The 

 small mouth opening is sub-pentagonal, and (fig. 2 b) situated near the anterior third; 

 the peristome is undulated, and the inter-ambulacral are smaller than the ambulacral 

 lobes ; the tubercles at the base are larger than those on the dorsal surface ; they are like- 

 wise fewer in number, and arranged with much irregularity on the plates (fig. 2 b). 



The apical disc is fortunately preserved in one of my specimens (fig. 2 ^) ; it is 

 formed of two small anterior ovarial plates, and two larger posterior ovarials ; the right 

 anterior plate supports the spongy madreporiform body ; between the posterior ovarials 

 the single imperforate ovarial is situated, composed of two pieces, an anterior and a 

 posterior half (fig. 2 h). This arrangement of the genital plates is common to all the 

 Hyboclypi with preserved discs I have examined ; I have therefore noted it as a character 

 common to the genus. The five ocular plates are small, and wedged between the 

 depressions formed by the ovarials ; their eye-holes are marginal, whilst the perforations 

 in the ovarials are situated near the border. 



The tubercles are small and numerous, each plate having three or four concentric rows 

 arranged in diagonal Jines on its surface (fig. 2y). As the dorsal portion of this pretty 

 little urchin varies considerably, I have given outlines in figs, c,/, i, k, of some of the 

 most remarkable deviations from what I consider as the typical form, fig. 2 d. 



Affinities and differences. — Hyboclypus caudatus differs from the other Oolitic species 

 in its more oblong form, and especially in having the single inter-ambulacrum developed 

 into a kind of caudate process ; by this character it is readily distinguished from H. gib- 

 berulus, Agassiz, and H. ovalis, Wright, which it otherwise resembles ; the mouth and 

 vent are likewise placed nearer the anterior border than in these allied forms. The sub- 

 orbicular shape which H. agariciformis invariably retains throughout its numerous varieties 

 of elevation and depression of the upper surface readily distinguish it from all the forms 

 of H. caudatus I have met with. 



Locality and Stratigrapliical position. — This is rather a rare urchin. I have found it 

 occasionally in the upper beds of the Inferior Oohte, " the Gryphgea Grit " of Leckhampton, 

 Birdlip, Shm'dington, and Ravensgate Hills, associated with Gryphcea sublobata, Desh., 

 Lima pecteniformis, Scloth., My opsis punctata, Buck., Cercomya rostralis, Wright, Tere- 

 bratula impressa, V. Buch. It occurs likewise in the Trigonia Grit at Hampen, associated 

 with Ammonites Parkinsoni, Sow., and the numerous other Mollusca and Echinida 

 which characterise that rich zone of life. 



