40 PEOr. p. M. DUNCAN AND ME. W. P. SLADEN ON 



the ambulacra close to the radial plate in both species, and 

 there are a few secondary tubercles in the abactinal ambu- 

 lacra in both species. The great tubercles are of the same 

 type, and the pairs of pores of the triplet are wide apart and 

 often intrude upon the flat edge of the base of a great tubercle. 

 The interradial plates are higher than in the fossil forms, and 

 there is no obliquity of the outer parts. The differences between 

 the recent and fossil ambulacra being iu matters of detail only, 

 it is important to study these structures in O. Maillardi, and to 

 compare them with those of the fossil species. We do not there- 

 fore propose to repeat any descriptions which can be found in 

 the work of A. Agassiz, but to offer those new structural details 

 which have been noticed by us. 



The Amhulacral Region at the Ambitus. — In zone " a " of 

 ambulacrum " IV." the fourth and fifth tubercles from the radial 

 plate are the large ones at the ambitus ; tlae third and the sixtli 

 are subequal and are decidedly smaller. The fourth and fifth are 

 the largest tubercles of zone " h," and the third tubercle is 

 slightly larger than that of the other zone. All the tubercles 

 have, in this part of the ambulacrum, an expanded thin edge and 

 almost circular base which covers nearly the whole of the com- 

 pound triple plate, leaving, however, space enough for a miliary 

 or two between the adoral edge of the fourth and the aboral edge 

 of the fifth plate ; but there is no such space between the fifth 

 and sixth plates, for their bases almost come in contact in 'con- 

 sequence of a slight downward projection of the bases, which just 

 at that spot are a little more sharply curved than elsewhere. 



There is also some space in the median area between these 

 great tubercles, but only sufficient to permit of there being some 

 miliaries and very small mammillated tubercles in a single row at 

 the median line, and a small group of the same kind of ornamen- 

 tation at the angles. It is this grouping that seems to produce the 

 slight departure from the circle of the curvature of the bases 

 noticed above. The shape of the tubercles is that of the fossil 

 forms : there is a broad-based boss sloping up without any tumidity 

 to a narrow top surmounted by a mamelon, which contrasts greatly 

 with the dimensions of the rest, and which is surrounded by a 

 shallow narrow groove, so that the neck of the mamelon is nearly 

 as broad as the mamelon itself. 



The largest of the tubercles in a medium-sized specimen is 

 4 millim. in breadth and the height is not quite 2 raillim, 



