38 W. KEEPING ON THE PALEOZOIC ECHINI. 



surrounded by a smooth ring. Ambulacral areas as broad as one 

 of the adjacent interauibulacral plates (5 mm.) ; composed of two 

 rows of plates, which are wedge-shaped, and granulated like the inter- 

 ambulacrals ; pores near the outer edges of the plates, bigeminal. 



The interambulacral plates overlap from below upwards (?) * ; and 

 the antero-lateral overlaps the postero-lateral edge f ; the ambula- 

 cral plates overlap in the opposite direction. As a consequence of 

 the alternating wedge-shaped ambulacral plates, only every other 

 one makes an impression on the interambulacral plates, which are 

 thus marked with 4—6 indentations. 



Near to P. gigas (McCoy), from which it differs in the greater 

 relative number of ambulacral plates, each of which has only two 

 instead of four pores, and in the overlapping of the plates. This 

 latter structure is not nearly so pronounced as in Perischodomus, 

 and is, indeed, of a different type. In that genus the plates are 

 thin and overlap like scales ; here they are thick and bevelled off as 

 a carpenter planes off his plank in joining (fig. 11) ; so that the test 

 here could only have possessed a slight degree of mobility, and would 

 scarcely be capable of " panting " X like its modern representative 

 Ccdveria. The specific name refers to the overlapping of the plates, 

 which is interesting as being of a character intermediate between the 

 simply opposed plates of ordinary Echini and those of Perischodomus. 



Loc. Hook Head, Pethard. 



Pal^echintjs gigas (McCoy). PI. III. figs. 12, 13. 



In this species the ambulacral plates seem to be welded in pairs. 

 They were described by Prof. McCoy § as each possessing two 

 pairs of pores (fig. 12) — a character which M. Desor || suggests might 

 separate it as a genus from Pcdcechinus ; but its true nature is 

 clearly shown by a specimen in a part of our collection arranged by 

 Prof. McCoy himself, where the double character is indicated by a 

 paij' of median interlocking articular angles to each plate (fig. 13). 

 This is the only example recorded of the fusion of the ambulacral 

 plates in the Palaeozoic Echini, a feature so common among recent 

 Urchins. 



Pal^chinus sph.ericus (McCoy). 



A specimen of this species shows a loose ocular plate with two 

 pores, agreeing with those described by Mr. Baily % in P. ellijoticus. 



* Since both poles are absent, I am unable to determine the direction of the 

 imbrication ; but we may presume that it was the same as in the other Perischo- 

 echinidae. 



t This character may be taken as generic ; and Messrs. Meek and Worthen 

 suggest that it is of "even more than generic importance" (Pall. 111. iii. p. 522). 

 But since in every characteristic, except the bevelling of the plate, it agrees with 

 Pal&chinus, it is perhaps best to retain it in that genus till we obtain further 



& VI Q &Y\ OP: 



J The Depths of the Sea, by Prof. Wyville Thomson, p. 156. 

 § Carb. Foss. Ireland, pi. xxiv. f. 4, p. 172. 

 || Desor, Synopsis des Echinides, p. 158. 

 1 Eoyal Geol. Soc. Ireland, 1864. 



