W. KEEPING ON THE PALEOZOIC ECHINI. 37 



moment that that genus is ultimately proved to possess imbricating 

 plates, the two genera Lepidechinus and Perischodomus may perhaps 

 be considered identical"*. Still I think there is abundant differ- 

 ence between the two genera. The interambulacral plates in Prof. 

 Hall's genus are far more numerous (as many as eleven ranges of 

 plates) ; and these dwindle in size towards the summit ; also, on the 

 upper surface at least, well-developed tubercles are present on all 

 the ranges of interambulacral plates f instead of being confined to 

 the two lateral ranges. 



The " imperfect specimens " mentioned by Messrs. Meek and 

 Worthen as "doubtless belonging to Lepidechinus " $, would pro- 

 bably have been referred by them to Perischodomus, had they known 

 of the imbricating character of the plates in this genus. Lepides- 

 t7ies§ is distinguished from this by its numerous (10) ranges of 

 ambulacral plates ; Archceocidaris by its large plates, each with a 

 primary tubercle. 



2. Ehoechinus II, nov. gen. 



Test rather small, spheroidal (?), interambulacral areas consisting of 

 five ranges of irregular plates, two of which disappear before 

 reaching the apical pole ; plates covered with small granules ; no 

 large tubercles. Ambulacral areas narrow, composed of two ranges 

 of interlocking plates, each perforated by a pair of pores. The 

 interambulacral plates imbricate from below upwards, and from the 

 central range outwards ; the ambulacral from above downwards. 



This genus is nearly allied to Perischodomus and Lepidechinus, 

 from both of which it is distinguished by the absence of primary 

 tubercles. 



E. ieregtjlaeis (Keeping). PI. III. figs. 6-8. 



A small Urchin (diam. about 25 mm.), depressed at the poles. 

 Interambulacral plates echinated, with little sharp granules (fig. 8). 

 The median range is the first to disappear towards the poles, being only 

 seen near the equator of the test ; another range disappears at about 

 two plates from the summit. Ambulacral areas sunk, narrow (2-3 

 mm.); plates transversely elongated, pentagonal, the pores situated 

 about the centre of the plate. There are 4 or 5 ambulacral plates 

 to one interambulacral. 



Loc. Hook Head, Co. "Wexford, Carboniferous Limestone. 



3. Paljechinus. 

 Pal-echinus? inteemeditjs (Keeping). PI. III. figs. 9-11. 



Test spheroidal ; interambulacral areas with five (?) ranges of 

 regular plates, outer ranges pentagonal, inner ranges hexagonal. 

 Surface of the plates uniformly granulated with small tubercles 



* Quart. Journ. Geol. Soc. vol. xxx. p. 313. 



t Prof, Hall, Twentieth Report, State Cabinet, New York, t. ix. fig. 10. 



\ Pal. 111. ii. p. 2U5. § Loc. cit. iii. p. 522. || pod, a pomegranate. 



