36 W. KEEPING ON THE PALEOZOIC ECHINI. 



from below upwards, and from the centres of the areas outwards ; 

 ambulacral from the dorsal side downwards. 



The specimen which has disclosed this structure is a flattened, 

 nearly perfect, dorsal half of P. biserialis (McCoy) *. 



The interambulacral plates are convex and irregular in form, as 

 shown in the figure given by McCoy t ; but those of the median row 

 are much more regular, being usually trapezoidal. The inferior over- 

 lapped surface of articulation in a well-exposed interambulacral plate 

 is seen to be about half the breadth of the exposed part of the plate 

 itself; it is not simply bevelled off flat, but is slightly hollowed 

 to receive the corresponding convex surface of the plate below 

 (fig. 3) ; on the contrary, in the lateral imbrication, the underlying 

 overlapped surface is slightly convex (fig. 2). 



The plates of the median range overlap those on each side of them 

 as well as those above, like the summit tiles of a roof. Of the five 

 ranges of interambulacral plates, two end suddenly at about the 

 hird plate from the apical disk, the last one being scarcely smaller 

 than the rest in the range ; the plates of the lateral ranges next- 

 above are larger than usual by way of compensation (fig. 4). 



The ambulacral areas are slightly sunk, their plates overlapped on 

 both sides by tbe interambulacral, and deeply interlocking with each 

 other. Like the interambulacral plates they are irregular, in places 

 club-shaped, the thick and thin ends alternating, in other places 

 more regular (as in McCoy's figure); their surface is minutely 

 granulated as in the interambulacral plates. 



Of the apical disk (fig. 4), the five genital plates are preserved in this 

 specimen. These are pentagonal in form, and are overlapped by the 

 interambulacral plates. They are perforated by numerous pores — 

 6 according to Prof. McCoy ; but one plate has 8 distinct perfora- 

 tions, while another almost as obviously could not have possessed 

 more than 6 ; so that in this genus we have the same remarkable 

 irregularity as is observed by Messrs. Meek and Worthen in the 

 genus Melonites %. 



Other small triangular plates are seen, which may possibly be the 

 ocular plates ; but I am unable to detect any perforations in them ; 

 moreover they are somewhat displaced, so as to be opposite the 

 interambulacral plates ; this displacement is shared by the other 

 apical plates. If these are the ocular plates, it is singular that they 

 should overlap the genital plates. Within the circle are seen other 

 plates, two of which appear to be in situ ; these may represent the 

 suranal series of the Saleniadse and Cidaridse. 



Two sets of spines were present in this species (P. biserialis). 

 Prof. McCoy described the primary one as cylindrical and smooth 

 ' ' as far as seen." Only the secondary ones (fig. 5) are preserved on this 

 specimen ; and these, under the microscope, are seen to be delicate, 

 needle-shaped, and coarsely ribbed. Length 3-5 mm. 



In its general aspect, and in the complex nature of its imbrication, 

 Prof. Hall's genus Lepidechinus approaches near to Perisclwdomus — 

 so much so that we find Mr. Etheridge saying, " presuming for one 



* Ann. & Mag. Nat. Hist. vol. iii. 1849, p. 254. f Loc. cit. p. 253. 



% Pal. Illinois, vol. ii. p. 228. 



