W, KEEPIXG OX THE PALEOZOIC ECHINI. 35 



2. Notes on the Paleozoic Echini. By Walter Keeping, Esq., of 

 the Woodwardian Museum, Scholar of Christ's College, Cam- 

 bridge. (Read June 9, 1875.) 



(Communicated by Professor T. McK. Hughes.) 



[Plate III.] 



The interest excited by the discovery, in 1869, of that remarkable 

 flexible Echinoderm, Asthenosoma or Cdlveria, in the deep Atlantic, 

 is increased by the fact noticed by Mr. J. Young * and other palaeon- 

 tologists, that certain other forms, whose day ended far back iu geolo- 

 gical time, also possessed the same overlapping plates and consequent 

 flexibility. This structure was first described by Prof. J. Hall in 

 Lepidechinus, and has since been noticed by Meek and Worthen in 

 another American genus, Lepidesthes, and by Mr. Young in the 

 scattered plates of Archceocidaris from our own Carboniferous rocks. 

 This character I have also detected in other genera of the same group 

 of Perischoechiniche from Co. Wexford, Ireland. 



It is worthy of note that whereas in the more modern (Creta- 

 ceous and Recent) group of flexible Echini (E chin othuri das) the inter - 

 ambulacral plates imbricate from above downwards, and the ambu- 

 lacral from below upwards, in these Palaeozoic forms (Perischoechi- 

 nidas) the opposite arrangement obtains. 



1. Perischodomus. PI. III. figs. 1-5. 



Since the first notice of the genus Perisehodomus by Prof. McCoy, in 

 the ' Annals of Natural History,' vol. iii. (1849) p. 251, this specimen, 

 which is one of McCoy's two types, seems to have been lost sight of 

 by palaeontologists ; for we find Mr. Etheridge, Jun., observing f that 

 " very little is known regarding this genus," and Meek and Worthen 

 say X " only a single very imperfect specimen of it has, we be- 

 lieve, yet been found." 



McCoy's diagnosis of this genus is : — " Spheroidal, depressed, sub- 

 pentagonal; ambulacra narrow, of two rows of small plates, most 

 usually of a transversely elongate pentagonal figure, and each 

 pierced by one pair of simple pores ; interambulacra wide, of five 

 rows of plates very irregular in size and shape, all the plates 

 covered with small equal granules or secondary tubercles, while the 

 rows on each side adjoining the ambulacra alone bear the small 

 smooth primary spines, one on each, the supporting tubercle being 

 small, mammillated, perforated but not crenulated, surrounded by a 

 double ring, and situated not in the centre but near the ambulacral 

 edge, a little above the middle ; ovarian plates pierced each with 6 

 foramina ; mouth and anus small, both central." 



To this must now be added : — interambulacral plates overlapping 



* Geol. Mag. vol. x. p. 302. t Quart. Journ. Greol. Soc. vol. xxx. p. 312. 



\ Pal. Illinois, vol. ii. p. 295. 



D 2 



