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



tubercles generally crowded towards the upper part of the plate, with secondary and 

 miliary tubercles towards the lower edge (PL XXV. fig. 3) ; the edges of the plates are, 

 however, left quite free from tubercles, the tubercles leaving bare lines to indicate the 

 sutures on each side of them below the rudimentary petals ; the tuberculation of the 

 ambulacral area is similar to that of the interambulacral areas, only the tubercles are 

 smaller. 



The apical system is compact ; the four genital openings large (PL XXV. fig. 5), placed 

 close together, well above the ocular plates ; the sutures of the genital plates are obhterated ; 

 the madreporic body extends into the posterior interambulacrum in a large horse-shoe- 

 shaped form, and extends also anteriorly between the four genital plates. 



The ocular plates are triangular, rounded at the apex, with a prominent pit in which 

 is placed the ocular pore. Seen from the interior, the apical sy.stem shows the great 

 development of the abactinal part of the calcareous canal (PL XXV. fig. 8) with the 

 slender ducts leading from the genital openings to the genital organs. 



The ambulacral pores round the actinostome give passage to tufted ambulacral 

 suckers ; the ambulacra are simple pores from those to the lower extremity of the 

 rudimentary petals formed hj the narrow more elongate amljulacral plates perforated by 

 pau's of pores ; these petals flare slightly at the lower extremity, the posterior lateral pair 

 flaring more widely and not extending quite so far towards the ambitus. The odd 

 anterior ambulacrum is simple, and is not petaloid towards the abactinal system. On the 

 abactinal side the primary spines of the interambulacral area are curved, moderately long, 

 the whole test thickly covered with them and the intervening mfliaries and secondaries. 

 On the actinal side the spines are somewhat less stout. The spines of the ambulacral 

 areas are smaller on the abactinal surface, and quite minute on the actinal side and in the 

 petaloid region of the ambulacra. 



The colour of the test when denuded is reddish-brown, the spines of a brownish- 

 yellow colour, with occasionally lighter-coloured spines. 



In younger specimens of Paleopneustes murrayi the test is quite flattened, much as 

 in Maretia proper ; this is contrary to what is generally the case among Spatangoids, 

 where, as in Brissopsis, Spatangus, Hemiaster, Schizaster, and young specimens of many 

 other genera, the test is quite globular, and subsequently becomes flattened or assumes the 

 outline of the adult ; it is, however, what we have found to be the case in the Ananchytid- 

 like genera such as Cystechinus, Urechinus, Spatagocystis, and also in Genicopatagus. 



In young specimens measuring about 30 and 40 mm. in length, the ambulacral petals 

 are quite straight, and do not flare at the extremity (PL XXXV.'' figs. 8, 9). The 

 peripetalous fasciole is very distinct, but very thin, a mere line having much the same 

 course as in older specimens such as are figured on Plate XXV. fig. 3, where it is also often 

 reduced to a mere thread, and perhaps eventually disappears, as is the case in the 

 Barbados' species {Paleopneustes cristata), in which we have no such fasciole, and in 



