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



(PI. XXXV.* fig. 2). The most striking feature perhaps of this genus is the small 

 number of coronal plates composing the test, particularly in the interambulacral areas. 

 This genus holds to the Spatangoids in that respect, much the same relation which 

 Cidaris holds to the normal Echiuids. The number of plates of the amliulaeral area is 

 comparatively small also, but not more so than we have found to be the case in Cyst- 

 echinus, compared with the number of interambulacral plates. 



In fact, in the latter there is less disproportion in the number of the plates of the two 

 areas, although the number of plates is larger, the ambulacral plates are proportionally 

 larger. The ambulacra are all identical in structure, the odd ambulacrum not differing 

 from the lateral ambulacra in structure, but in having a less number of small plates with 

 double pores, the double pores giving to the ambulacra above the ambitus the least 

 possible petaloid appearance much as in Argojyatagus. The apical system is more like 

 that of Cardiaster, not being so elongated as in Holaster ; there are four large adjacent in- 

 terambulacral plates occupying the whole of the apical system. It has, like Cardiaster and 

 Holaster, a very prominently labiate actinostome ; the position of the anal system is like 

 that of Toxaster, while the flat actinal surface and the globular outline remind one of 

 Cardiaster ; the actinostome is more central than in that genus. Like the typical 

 Paleopneustes this species possesses no fascioles. This might perhaps be called an 

 eminently Galeritid Spatangoid. 



*Genicoixaagus affinis (PI. XXXI. figs. 12-22 ; PL XXXV.'' figs. 1-4 ; PI. XXXIX. 

 fig. 20 ; PI. XLI. figs. 38, 39 ; PL XLIII. fig. 18 ; PL XLV. figs. 20-24). 

 Genicopatagus afinis, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv. p. 210. 



In this species the primary tubercles on the abactinal side of the test are irregularly 

 placed on the interambulacral plates, occupying more the central portion of the plates 

 (PL XXXV." figs. 2, 4), the secondary tubercles scattered between them are somewhat 

 more numerous than the primaries. On the ambulacral plates there are from one to three 

 or four minute primary tubercles (PL XXXV.'' figs. 1,2, 4) with a corresponding number 

 of miliaries according to the size of the plates. The spines are straight, cylindrical, 

 rather short, larger on the actinal side (PL XXXI. figs. 12, 13), where the primary 

 tubercles are larger and are arranged in somewhat regular transverse rows on the 

 interambulacral plates (PL XXXV.'' fig. 2),^ The am1)ulacral j)lates of the actinal 

 surface are bare, carrying only miliary or few secondary tubercles ; the posterior 

 lateral ambulacra form wider bare areas than the others which are somewhat narrower 

 (PL XXXV.'^ fig. 1). 



The colouring of this species is very various, some of the specimens were violet, 

 while others from the same locality were of a dirty yellowish-green. One of the yellowish- 



1 Ooster has figured the spines of alliiid fossil genera of Holasteridte, which show a great similarity to those of 

 Genicopatagus. 



