REPORT ON THE ECHINOIDEA. 83 



are short, flaring at the extremity, of a greenish colour, banded with white and brown, 

 or with violet brown transverse bands. These are of a very uniform size Ijoth over the 

 ambulacral and interambulacral areas of the actinal surface ; these curved spines extend 

 only to the edge of the ambitus, where they are replaced by short sharp spines (PI. XVI. 

 fig. 6a-c) covered by a muscular sheath, the extension of the cuticle of the test (PI. XVI. 

 figs. 2, 3, 4, 7-9) which in the shorter spines of the ambulacral area and of the median inter- 

 ambulacral zone (PI. XVI. figs. 7-9) forms a simple bag at its extremity, while in the 

 somewhat longer spines both of the edge of the ambulacral and of the interambula- 

 cral zones the sheath is constricted in several places according to the length of the spines 

 (PI. XVI. figs. 2-4), and is frequently banded with transverse patches of colour. 



The whole test both on the actinal and abactinal sides carries short, sharp, slender 

 miliary spines (PL XVI. fig. 5), similar in structure to the longer spines of the abactinal 

 surface (PI. XVI. fig. 6.) This extension of the muscular sheath of the test over the 

 spine or a portion of the spine is characteristic of those spines of Echinothuridse in which, 

 owing to the absence of the milled ring proper as in other Echinids, the spines have retained 

 a more or less embryonic structure, and the muscular belt which, starting from the edge of 

 the scrobicular circle in Echinoids, generally terminates at the milled ring, extends in 

 some species of this family along the shaft of the spines either along a part of the shaft 

 or beyond its tip. This is a feature which is eminently characteristic of all young Echinids 

 which I have had occasion to examine. It reminds us also of the mode of growth of the 

 pedicellariae as a protuberance of the calcareous test covered by the general cuticle 

 covering the whole test, which eventually forces its way through this as also do the young 

 spines of Echinids, while in some of the Echinothuridse this sheath remains permanently 

 growing with the growth of the spines of the abactinal region. In other species of the 

 group {Asthenosoma coriaceum) this cuticle extends over the miliary tubercles with rudi- 

 mentary spines, leaving small pits which are more or less regularly arranged along the 

 coronal plates of the test. These spines difi"er materially from the primary spines of the 

 Perischoechinidte, which so far as is known are provided with a milled ring. The tubercles 

 also are both perforate, imperforate and not crenulate, difi"ering in this respect from those 

 of the Diadematidse proper. Towards the abactinal system and over the actinal system 

 the integument of the test is so thick as to conceal completely the sutures of the plates. 

 The elongated narrow actinal plates are remarkable (PI. XVII. fig. 4) for the arrangement 

 of the small tubercles they carry in regular horizontal rows concentric with the test ; the 

 pair of pores, the continuation of the poriferous zone, are placed in the centre of each 

 actinal plate. 



In the abactinal system (PI. XVII. figs. 2, 3) the ocular and genital plates, with the 

 exception of the madreporic body, are indicated when denuded of spines by a large, some- 

 what indistinct plate, smooth towards the ocular or genital pore and covered with coarse 

 granulation at the anal edge. The anal system itself is covered by coarse distant granu- 



