84 THE VOYAGE OF H.M.S. CHALLENGER. 



lation. The genital plate carrying the madreporic body is raised round the edges, 

 depressed in the centre, and extremely prominent. The anal opening is in the centre of 

 the anal system in the middle of a small ring formed by the rising of the integument of 

 the anal system. On the actinal side the large primaries of the interambulacral areas 

 are arranged in horizontal rows (PI. XYII. fig. 1), well separated by irregular groups of 

 secondaries and miliaries, the tuberculation is quite regular in size over the whole of the 

 interambulacral areas. In the ambulacral areas there are from two to three primary 

 tubercles on each plate, between the poriferous zone and the median line separated by one 

 or two miliaries or secondaries (PI. XVII. figs. 1, 6), with a horizontal line of secondaries 

 separating the rows of pores towards the outer edge of the plates and a cluster of the 

 smaller tubercles near the median angle of the ambulacral plates. 



On the abactinal side the coronal plates become very gradually narrower and narrower 

 as they extend from the ambitus towards the abactinal system, and at the same time 

 the integument of the test separating the plates increases in width; the primary 

 tubercles diminish gradually in size and distinctness (PI. XVII. fig. 2) until towards the 

 abactinal system the tubercles are reduced to indistinct granules (PI. XVII. fig. 3), form- 

 ing more or less regularly curved narrow bands, with a re-entering curve towards the 

 median line, indicating the position of the interambulacral coronal plates (PI. XVII. 

 fig. 2). 



The same takes place in the median ambulacral spaces, but as the tubercles are 

 originally smaller the change takes place nearer the ambitus. In the median ambulacral 

 line a bare band separates the tubercles of adjoining plates, and a similar bare band 

 separates the outer row of pores from the interambulacral plates. The poriferous zone is 

 broad at the ambitus : it is broader than the corresponding part of the ambulacral plate. 

 It diminishes very gradually in width towards the abactinal system, and forms on the 

 abactinal side of the test three vertical rows of pairs of pores (PI. XVII. fig. 2). The two 

 inner rows are placed quite close together, the outer well separated from the two inner 

 rows (PL XVII. fig. 5) and further distinguished by a vertical row of small secondary 

 tubercles running between it and the two rows. On the actinal side these three rows 

 can be distinguished only near the ambitus (PI. XVII. fig. 1) ; they soon become ex- 

 tremely irregular by the gradual narrowing of the poriferous zone, so that about half-way 

 between the ambitus and the actinal edge the three rows are united (PI. XVII. fig. 6), the 

 pairs of pores having become contiguous and forming a slightly oblique row of three pairs 

 of pores. 



This species is probably the same which Grube first described as Asthenosoma variuin. 

 His original description was, however, so short that Echinologists did not recognise the 

 importance of the discovery of the genus, and Thomson himself when he first mentions 

 Calveria in the Depths of the Sea could not suspect its identity with Astheno- 

 soma, as Calveria does not possess the peculiar sheathed spines so characteristic of 



