34 FOSSIL ASTEROIDEA. 



The abactinal area of the disk within the boundary of the marginal plates is 

 covered with small sub-regular plates or paxillar tabula, an hexagonal form pre- 

 dominating especially in the radial regions ; and a small but distinct diminution in 

 size takes place as the plates approach the margin of the disk. All the plates 

 have their surface marked with minute, shallow, and closely placed punctations — 

 the impressions of the attachment of the granules previously present. The 

 primary basal plates are larger than any of the other abactinal plates, and they 

 are well shown in sevei^al of the drawings illustrative of this species (see PL X, 

 figs. 1, 2 a). Occasional plates bear small entrenched pedicellarise, the normal 

 form consisting of a small, central, lipped foramen with a lateral trench on each 

 side. It frequently happens, however, that the organ exhibits a more complex 

 development, and assumes a stellate form in consequence of the presence of addi- 

 tional trenches — five or six being not an unusual number — which radiate from the 

 central foramen ; the whole being placed on a small hemispherical elevation, and 

 producing an appearance shown on PI. X, fig. 2 d. 



The madreporiform body is large and subtriangular in outline ; and its surface 

 is sculptured by very fine striations which radiate from the centre to the margin, 

 with more or less wavy lines here and there (see PI. X, fig. 2 c). The margin 

 of the plate is surrounded by three large plates, one on the adcentral side of 

 the madreporite towards which it presents a straight suture; the other two 

 plates are on the remaining sides of the triangular body, and they have a 

 concave curve directed towards the madreporite to correspond with the 

 convexity of its sides. The position of the madreporite is nearer the centre of 

 the disk than the margin. 



The infero-marginal plates are seven in number, counting from the median 

 interradial line to the extremity — that is to say, there are fourteen for the whole 

 side of the disk, as against eight in the supero-marginal series. The length of 

 the three innermost plates on each side of the median interradial line is equal to 

 that of the superior series, but there are four infero-marginal plates corresponding 

 to the large ultimate supero-marginal. As seen in the lateral wall of the disk the 

 height of the infero-marginal plates is less than that of the supero-marginal series. 

 The breadth of these plates adjacent to the median interradial line on the actinal 

 surface is 7*5 mm. in an example whose major I'adius measures 36'5 mm. and the 

 minor radius 27"5 mm. The breadth of the marginal border rapidly diminishes 

 towards the extremity of the ray. The surface of the plates is ornamented in a 

 precisely similar manner to that of the supero-marginal plates. A narrow border 

 of smaller granulation is also present round the whole margin of the plate, 

 similar in all respects to that already described in the case of the superior 

 series of plates. 



The adambulacral plates are small, about or nearly twice as broad as long, and 



