FROM THE CORAL RAG. 285 



The inter-ambulacral areas are four times as wide as the ambulacral ; there are about 

 thirty-two pairs of plates in each column, of which eighteen belong to the upper surface, 

 and fourteen to the base ; the plates are bent in the middle, and at the widest part of 

 the area, each plate supports seven tubercles ; the four nearest the zones are arranged in 

 oblique pairs, and the three nearest the centro-suture in horizontal lines (fig. 2d); at 

 the margin there are from eighteen to twenty tubercles in one horizontal series. 



The tubercles on the upper surface are small (fig. 2 a) ; they have slightly sunken 

 areolas, and are raised upon small bosses with smooth summits (fig. 2/) ; the areolas are 

 surrounded by six small granules (fig. 2/), and others are scattered sparingly on the 

 surface of the plates; these granules, like the tubercles, are all perforated (fig. 2d). 



The tubercles at the base are much larger (fig. 2 b), and they are arranged in close- 

 set, horizontal lines ; the areolas are more excavated, and rather square-shaped ; they are 

 bounded by a single row of granules (fig. 2 g) ; and the tubercles of both areas are about 

 the same size. 



The base is concave, slightly so at the sides, but much depressed at the centre ; the 

 mouth opening is small, and the peristome is equally decagonal. " Its circumference is 

 armed with ten prominent carinas, which, in the interior moulds, have left on the borders 

 of the inter-ambulacral areas very apparent imprints." * These carinas, which M. Desorf 

 had already observed in a specimen of Pygaster costellatus, are doubtless destined in 

 the Pygasters to replace the auricles of the Cidarid7E and Clypeasterida. M. Michelin^ 

 has recently discovered in a Pygaster umbrella a masticating apparatus, which resembles 

 that of a Clgpeaster, the united pieces of which form a very acute pyramid. 



The anal opening is very large, and occupies more than the upper half of the single 

 inter-ambulacrum (fig. 2 a) ; it has a pyriform figure, contracted above and swelling out 

 below, which, with the discal space, produces a keyhole-like opening in the test. In most 

 of the specimens this aperture is not symmetrical, and bulges more to the left than to the 

 right side ; in fig. 2 a a portion of the test is broken, which gives the vent in this specimen 

 a more symmetrical form than in all the others I have examined (fig. 2 c). 



The greater portion of the apical disc is preserved in one of the specimens from Malton, 

 kindly given me by Dr. Murray (fig. 2 b, e) ; the genital plates have an irregular rhomboidal 

 form, with pointed apices, and perforations for the canals near the apex ; the right antero- 

 lateral plate is disproportionately large, and the shield-like madreporiform body extends back- 

 wards into the centre of the disc (fig. 2 e) ; the ocular plates are small triangular bodies 

 wedged into the angles formed by the genitals ; the eye-holes are marginal and lodged in a 

 depression between the plate and the apex of the ambulacral area (fig. 2 e) ; the surface of 



* Cottean, ' Etudes sur les Echinides Foss.,' p. 197. 

 f 'Monogr. des Galerites,' p. 76. 



X Hardouin Michelin, Description de quelques nouvelles especes d'Echinodermes fossiles, ' Revue et 

 Magasin de Zoologie,' 2 e serie, torn, v, p. 36, 1853. 



