MITRASTER HUNTERI. 61 



example preserved in the British iluseum bearing the register-number " 46,772," 

 in nhicb 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 " E 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. The 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 jilain, 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 PI. XII, fig. 3 d). A narrow depressed 

 border surrounds the entire margin of each plate, which is very minutely 

 punctate. 



The adambulacral plates, wliich 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 



