316 COLLYRITES 



an important guide to its determination, and this character is very accurately delineated in 

 Leske's original figure of his Spatangites ovalis ; the apices of the postero-lateral ambulacra 

 converge on the back at the distance of three fifths of an inch behind the antero-lateral pair. 



The poriferous zones are narrow, and in consequence of the depth of the ambulaoral 

 plates the pores are wide apart ; each plate is perforated with a pair of holes near its 

 lower border (fig. 1 e), and there are from two to three ambulacral plates opposite one 

 inter-ambulacral. 



The pores are disposed in oblique pairs, they are very distinct on the sides and back 

 fig. 1 a, c, d), but are small and indistinct at the base (fig. 1 b). 



The inter-ambulacral plates are large and bent, each plate forming a double inclined 

 (plane (fig. 1 e). The anterior are much narrower than the posterior areas ; they are 

 iniiformly smooth and gibbous on the sides ; the single inter-ambulacrum is bevelled 

 obli(piely, and slightly flattened at the upper part of the posterior border ; at the extreme 

 upper part of this region, and immediately beneath the centre of the arch formed by 

 the posterior ambulacra, the anal opening is situated (fig. 1 a, c,d). It has a pyriform 

 shape, with the apex directed upwards, and terminates between the apices of the posterior 

 ambulacra. From its sides two obtuse ridges pass downwards and outwards, which 

 gradually- disappear about the middle of the area, near the anal opening; the tubercles are 

 larger, and set closer together, than at any other part of the upper surface (fig. 1 e). 

 The basal portion of the inter-ambulacrum is more gibbous and produced than the 

 corresponding parts of the other inter-ambulacra. 



The base is convex, with little or no undulation ; anteriorly there is a slight concavity, 

 and posteriorly an increased convexity, occasioned by the gibbosity of the single inter- 

 ambulacrum (fig. 1 b, d). The small mouth-opening is lodged in a slight depression, 

 situated near the anterior fourth of the antero-posterior diameter of the test, but the 

 precise relative situation of this aperture appears to vary a little in different individuals. 

 The peristome is sub-pentagonal, and in one specimen, appears to have rudimentary 

 notches, which would imply that the CoUpitida possessed jaws, but the organic evidence is 

 too feebly developed to enable me to state that such is unquestionably the case, unless 

 confirmed by a similar inequality of the margin in other specimens. 



The apical disc is situated behind the sub-triangular vertex, and therefore occupies the 

 anterior third of the back; it has a lengthened rhomboidal figure, and is formed very 

 much like the disc of C. rivgens, already described in detail. (See page 311.) 



The single ocular plate is small, and occupies nearly the vertex, having behind it 

 the two anterior ovarial plates, which are situated between the single and the antero- 

 lateral ambulacra, the left plate being placed further forward than the right plate, which 

 supports the madreporiform body. Behind the anterior ovarial are the two large anterior 

 pair of ocular plates, and immediately behind them are the posterior ovarials (fig. 2/). 

 The position of the posterior pair of ocular plates I have not ascertained; if they exist at 

 the summit of the posterior ambulacra, their true homological position, they are so 



