Vol. 5S-] 



ECHINOIDEA AND OPHITJEOILEA. 



709 



curvature of the ambulacrum, which removed it from the protecting 

 influence of a thin layer of sandstone that covered it elsewhere. 



Fig. 14. — Transverse sections through the ambulacrum of a specimen 

 of Echinocystis pomum partly preserved in carbonate of lime. 







On grinding deeper, evidence was obtained in favour of this explana- 

 tion, so that on the whole the existence of an inner series would 

 appear to be indicated. 



Protocidarts sp. 



I had completed an account of this fossil when the last volume 

 (vol. lii) issued by the Palseontographical Society reached me : and I 

 found very similar remains described there by my friend Mr. Whid- 

 borne, under the name of Protocidaris acuaria, gen. efc sp. nov.^ 

 These are found in Devonian rocks, east of Barnstaple. Like many of 

 the fossils from Leintwardine, the specimen in the Oxford University 

 Museum (Grindrod Collection) presents casts of the original structures, 

 more or less iron -stained, but in some cases preserving some of the 

 original calcareous substance. Spines and interambulacral plates are 

 all that remain of the organism, which most probably was an Echinoid. 

 The plates are thin, irregularly oval or round, measuring 3x5 mm. 

 in some instances ; they are minutely granulated and bear a widely- 

 perforated central tubercle, which does not appear to have been seated 

 on a boss. The spines are of two kinds, a smaller, 2 or 3 mm. long, 

 and a larger, as much as 13 mm. long, truncated at the head, which 

 is deeply and widely excavated like the mamelon ; no longitudinal 

 striations are now visible. 



It would not be prudent to give these remains a specific name, 

 for they may belong to the same species as that described by 

 Mr. Whidborne ; the only points of difference now discoverable are 

 the absence of a boss and of striations on the large spicules. 



On closer examination of Echinocystis^ a marked resemblance 

 between the interambulacral plates of this genus and those of 



lorn. cit. (1898) p. 203 & pi. xxv, figs. 1-2 a. 



