116 THE FOSSIL ECHINOIDEA 



Family GLYPHOSTOMATA. 



Subfamily DIADEMATIB^. 



Genus CYPHOSOMA, Agassiz, 1840. 



(See anted,, p. 31, Part ii.) 



1. Cyphosoma mackostoma, Duncan & Sladen. Plate XXII, Eigs. 8-10. 



Test of moderate size, circular in marginal contour, much depressed, tlie diameter 

 being more than twice and a half the height, flatly convex above, slightly concave 

 beneath, not greatly impressed around the peristome, which is large ; margin rounded 

 and tumid. 



Ambulacral areas comparatively wide, more than half the breadth of the interambu- 

 lacral area. Poriferous zones straight, or with a slight tendency to flexure ; pores 

 arranged in simple pairs, excepting at the peristome, where there is a little crowding, 

 and a few biserial pores or even arcs of three are produced. The pores of a pair are 

 separated by a slight eminence, and a costal ridge is produced from this along the upper 

 margin of the inner pore, by which this is separated from the neighbouring pore of the 

 succeeding pair. The ridge is not prolonged between the outer series of pores, and at 

 its inner extremity usually terminates in a granule on the ambulacral plate. The inner 

 pore is transversely elongate, larger than the circular outer pore, and is excavated out 

 of the ad oral margin of the plate. There are four, or occasionally five, poriferous plates 

 to each whole ambulacral plate. The interporiferous areas have two rows of large 

 primary tubercles, 14 or 15 in each series; these tubercles are crenulate and imperfo- 

 rate and occupy most of the plate, and are accompanied by a few unequal-sized and 

 irregularly-placed granules, a more or less definite row often occurring on the aboral 

 margin of the plate. None of the granules can be said to rank as secondary tubercles. 



Interambulacral areas with two rows of large primary tubercles, 12 or 13 in each, 

 very slightly larger than those of the ambulacral areas, which they resemble in every 

 respect, and diminish slightly in size towards the apical extremity of the series. The 

 primary tubercles are placed rather nearer to the median interambulacral line than the 

 middle of the plate. Between the primary tubercle and the poriferous zone are two 

 rather large secondary tubercles, one above the other and slightly oblique, the upper- 

 most being rather nearer to the primary tubercle. This double series does not extend 

 very far beyond the ambitus on the abactinal surface, but is continuous up to the peri- 

 stome, the tubercles being considerably diminished in size. Several small and irregu- 

 larly-placed granules are present on either side of the secondary tubercles, and a few 

 similar and unequal-sized granules occupy the inner portion of the plate between the 

 primary tubercle and the median suture, one standing near the inner extremity of the 

 aboral margin being usually the largest in the neighbourhood of the ambitus, but much 

 too insignificant to be ranked as a secondary tubercle. The miliary zone along the 



