498 TOXOPNEUSTES PILEOLUS. 



vertical rows of pores are placed slightly closer together, and separated from 

 the inner row. In specimens where there are at the ambitus twelve vertical 

 rows of tubercles in the interambulacral space, there are eight in the am- 

 bulacral region. Only one vertical row on each side of these regions extends 

 to the abactinal system ; the others extend some ways above the ambitus, 

 leaving a small part of both the median areas bare, in proportion to the 

 development of the vertical rows. The miliaries and secondaries are arranged 

 in horizontal rows across and between the primary vertical rows, forming a 

 simple network round the scrobicular circle of the primaries. 



The abactinal system is compact; the madreporic genital very prominent; 

 the genital openings large near the apex of the plates; the genital and 

 ocular plates carry one large tubercle, with secondaries and miliaries ar- 

 ranged round the scrobicular circle as round the primaries of test. The 

 anal plates are small, smooth, oblong, with a few larger triangular ones on 

 the side opposite the anus. The spines above the ambitus are short, mod- 

 erately stout ; those of the lower side, however, are much longer and more 

 slender. The outline of test from above is pentagonal, with re-entering 

 angle in the median interambulacral space; the ambulacra projecting beyond 

 the general outline of the test. In profile the outline is generally somewhat 

 . conical, though young specimens are more regularly arched, and, when quite 

 small, almost globular ; in older, fully grown specimens the shape is some- 

 times quite globular. The actinostome is more or less sunken, and the 

 lower surface concave ; in medium-sized and large specimens the depth of 

 the actinal cuts varies considerably. 



The extensive series of specimens of this species in the Jardin des Plantes, 

 and a number of specimens from Mauritius in the Museum Collection, show 

 such variations that I am unable in the adult to distinguish as distinct 

 species what I had called Boletia rosea from Central America, and the com- 

 mon Indo-Pacific, B. pileolus. Though, if what Mr. Yen-ill has called Psam- 

 mechinus pictus should turn out to be the young of B. rosea (B. picta), as he 

 is inclined now to consider it. it would be quite easy to separate these two 

 species when young by an examination of the buccal membrane, which is 

 stoutly imbricated in P. pictus, while the actinal membrane of young 

 specimens of B. pileolus, of the same size, is particularly bare and free from 

 limestone plates, except in the vicinity of the two large buccal plates. 



