FROM THE INPERTOR OOLITE. 311 



pair describe three curves in their course between the mouth and the disc (fig. 3 c) ; 

 the three anterior ambulacra converge nearly in the centre of the back, at the front, 

 and sides of the apical disc (fig. 3 a, i). The posterior pair are somewhat wider than the 

 others, they curve gracefully round the single inter-ambulacrum (fig. 3 c), form an arch 

 romid its produced and truncated border, and converge above the anal opening (fig. 3 d) ; 

 the anibulacral areas are formed of small plates, of which, on the dorsal surface, there are 

 four opposite one inter-ambulacral plate (fig. 3 e), but at the base there are only about 

 three in the same space (fig. 3/). Each ainbulacral plate is perforated at its outer side 

 with two very small pores, placed obliquely across the zones (fig. 3 e), which are narrow, 

 and observed with difficulty. Near the mouth the ambulacra widen, and the pairs 

 of pores are disposed in about three oblique series of three pairs in each (fig. 3 b); as 

 the plates are closely soldered together, their relation to the pores in this region is 

 very indistinct. 



The inter-ambulacral areas are of unequal width ; on the upper surface they are on 

 a level with the ambulacra, and with them form a uniform, convex back, but at the base 

 they are extremely prominent and nodulated ; so much so, that the ventral is as 

 remarkable for its midulations as the dorsal is for the smoothness of its surface. The 

 single inter-ambulacrum differs from the others in being produced posteriorly, and 

 is extremely prominent and gibbous inferiorly (fig. 3 b, c). 



All the plates are covered with minute, perforated tubercles, raised on uncrenulated 

 bosses, and surrounded by sunken areolas (fig. 3 y) ; the inter- tubercular surface is 

 so finely granulated that the tubercles appear conspicuous only when examined with a 

 lens. The tubercles are most numerous at the base ; the spines are unknown. 



The apical disc is a curious structure, it is composed of two pairs of perforated ovarial 

 plates (fig. 3 h, i), disposed in pairs at some distance apart, and separated by three 

 largely developed ocular plates, which extend into the centre of the disc ; the anterior 

 ovarials are of an irregular shape, and separate the single ambulacrum from the antero- 

 lateral ambulaci'a. On the surface of the right plate is the spongy, prominent, 

 madreporiforni body ; behind and between them in the median line, is a small diamond- 

 shaped plate, its anterior angle unites with the apex of the single ambulacrum, and 

 its posterior border with the anterior ovarials ; behind these are two rhomboidal-shaped 

 plates, which articulate before with the anterior ovarials, laterally with the apices of 

 the antero-lateral ambulacra, and behind with the posterior ovarial plates. Near the 

 points of junction of these plates with the ambulacra, the three small eyeholes are 

 situated, and behind the rhomboidal ocular plates the small, oblong, posterior ovarials are 

 placed. I have failed to discover ocular plates at the summits of the posterior ambulacra 

 in this species. The specimen which furnished these details has been mislaid, and could 

 not be found in time for the artist, but a very similar type of structure exists in 

 Colhjrites ovalis (PL XXIII, fig. 3/), where it is accurately drawn. The disc, it is 

 right to state, was studied with the microscope under an inch object-glass. 



41 



