MITRASTER HUNTERI. 61 
example preserved in the British Museum bearing the register-number “ 46,772,” 
in which the inner surface of the abactinal floor is exposed. 
The madreporiform body, in the examples in which I have detected its 
presence, appears to be small and subtriangular, and is marked with very coarse 
striations. 
The infero-marginal plates are five in number, counting from the median inter- 
radial line to the extremity,—that is to say, there are ten for the whole side of the 
disk as against six in the supero-marginal series. ‘The length of the two innermost 
plates on each side of the median interradial line is a little greater than that of 
the corresponding superior plates; the breadth of the plate adjacent to the 
median interradial line is 7 mm. in the specimen bearing the British Museum 
register-number “46,766,” and is a little greater than the breadth of the corre- 
sponding superior plate as seen in the example bearing the British Museum register 
number ‘ H 2583.” 
The second plate, counting from the median interradial line, is a little less 
broad, and the third is slightly more diminished in breadth, and its adcentral 
margin merges with a sweeping curve into the lateral distal margin, which gives 
the plate a more or less cuneiform shape. A large portion of the distal lateral 
margin of this plate abuts on the adambulacral plates. The fourth infero-marginal 
plate is very small and triangular in form, with the apex directed adcentrally, 
and with one side abutting entirely on the adambulacral plates. The fifth plate is 
smaller still. In consequence of the triangular shape of the fourth and fifth 
plates the greatest length of the third plate is opposite the apex of the fourth 
plate, and the length of the third plate gradually diminishes up to the outer 
margin. ‘I'he breadth of the third plate is 5-6 mm., and that of the fourth plate 
is only 3°2 mm. 
The surface of the infero-marginal plates, which is more or less plain, is orna- 
mented with small, widely spaced,and more or less equidistant punctations, and 
there is no trace of the tubercular mammillation present on the surface of the 
supero-marginal plates. On the inner third of the plate the punctations are more 
closely placed, and they have the appearance of falling into a more or less 
distinct reticulated arrangement (see Pl. XII, fig. 3d). A narrow depressed 
border surrounds the entire margin of each plate, which is very minutely 
punctate. | 
The adambulacral plates, which are small, are broader than long, and have 
their surface traversed by several ridges placed slightly obliquely, but I am unable 
to define the armature. 
The actinal intermediate plates, which are somewhat large in relation to the 
size of the disk, are rhomboid or polygonal in outline. Their surface is covered 
with small, closely crowded, uniform pits, upon which miliary granules or spinelets 
9 
