24 



are found, the largest one mostly placed in the inner corner of the plate. 

 Between the pores some small tubercles are found. The uppermost plates are 

 quite naked. 



In the character of the actinal side this form differs very much from 

 Astropyga and, indeed, from all other Diadematids. The large tubercles only reach 

 just below the ambitus; then they suddenly diminish very much in size, and from 

 here the whole actinal side is covered by a very close and fine, uniform 

 granulation, through which the limits of the plates cannot be traced. The 

 tubercles forming this granulation are, however, not of the same size; some are a 

 little larger, with distinct scrobicular area; the others are quite small miliary tubercles 

 filling up all the spaces between the larger ones. In the outer part of the granulated 

 portion there are, in both areas, indications of an arrangement of the tubercles in 

 longitudinal series, in continuation of the series of large tubercles at the ambitus, 

 but this arrangement is soon lost in the uniform granulation. At the edge of the 

 peristome the test is bent strongly inwards, the ambulacral areas being somewhat 

 sunken below the interambulacral areas. In the granulated part of the ambulacral 

 areas the pores are very small, scarcely half as large as those at the ambitus and 

 on the abactinal side; towards the apical sy.stem they are much larger; in smaller 

 specimens the size of the pores is almost the same on both sides. In the outer part of 

 the granulated portion in large specimens they are closely and irregularly arranged; 

 in the inner part, from about halfway to the peristome, they are 

 arranged in a single, almost straight row, the pores being wide apart 

 from each other, an arrangement the more curious as otherwise the pores are 

 usually very crowded at the edge of the peristome. Where the arrangement of the 

 pores in a single series commences the ambulacral area is a little widened and from 

 here again it becomes gradually narrower towards the peristome. 



An examination of the interior of the test gives a clear view of the struc- 

 ture of the abactinal ambulacral plates; they are constructed after the diademaloid 

 type and they are not arranged in compound plates of two triplets as in Astropyga^). 

 On the actinal side the structure cannot be seen distinctly in large specimens, 

 partly on account of the plates overlapping each other irregularly, and not adorally 

 alone. As the pores are so wide apart from each other, one might suppose it to be 

 the case here as in Kamptosoma that some of the primary components of the plates 

 had disappeared. That such is not the case, however, is sufficiently shown 

 by the radial watercanal which has no rudimentary branches on it; all the 

 branches and their ampullæ and tubefeet are well developed; but in the proximal 

 part they are wide apart ; whereas farther out they are very crowded. The plates are 

 thus composed of the usual primary components, but these are excessively large. 



M Comp. Duncan: On the Anatomy of the Aml)uhicia of the recent Diademiitiihe. (.). I, inn. Soe. Zoo- 

 logy. XIX. 1885.J 



