200 DEVONIAN FAUNA. 



below ; ambulacral pores two in each piece, and nearly central. Anal opening 

 and apical disc unknown. Jaws well developed. Entire surface ornamented with 

 numerous very small granules of uniform size, probably for the articulation of 

 minute spines, as in PalsRchinus. ,n 



The species described below seems so nearly to fall within the limits of the 

 above description that in the crushed condition of our fossils it hardly seems safe 

 to form a new genus for it at present, especially as the generic definition gathered 

 from a single specimen of a single species may perhaps require some modification. 



The chief particulars in which our species disagrees are — (1) that the inter- 

 ainbulacral plates bear six or eight irregular small tubercles of different sizes ; (2) 

 that the ambulacral plates seem smooth ; and (3) that there seem to be very 

 numerous minute acicular spines, mixed with a comparatively few larger ones. It 

 must here be distinctly observed that it fails to meet accurately the requirements, 

 not only of the genus, but of the family. 



Its imbricated plates, together with their large numbers in both areas, separate 

 it from all the other genera of this order mentioned by Zittel except Pholido- 

 cidaris? which differs among other things in the much larger comparative size of 

 the adambulacral plates, and in many of the interambulacral plates bearing a 

 large central tubercle. 



Perischodomus 3 has only two rows of ambulacral plates in each area. 



Hybechinus, of Meek and Worthen; 1 chiefly differs from Lepidesthes in having 

 the imbrication exactly opposite, i. e. from above downwards in the inter- 

 ambulacral zones, and from below upwards in the ambulacral (so that their lower 

 part is covered). Its interambulacral plates are rhombic instead of being 

 hexagonal, as in our species. The granules seem very much more minute; they 

 are not visible in the drawing of H. spectabilis, Worthen and Miller, 5 the type 

 species. 



1. Lepidesthes ? devonicans, Whidbome. Plate XXIV. figs. 1—2 ; and Plate XXV, 



figs. 3 a — -/. 



189G. Lepidesthes? devonicans, Whidborne. Proc. Geo]. Assoc, vol. xiv, p. 376. 



Description. — Test very large, regular, composed of very numerous plates, 

 which are approximately equal in height and breadth. Interambulacral areas with 



1 1868, Meek and Worthen, ' Geol. Surv. Illinois,' vol. iii, p. 522 (slightly abbreviated). 



2 1873, Ibid., vol. v, p. 510. 



a 1849, M'Coy, 'AnD. Mag. Nat. Hist.,' ser. 2, vol. iii, p. 254. 

 4 1883, Worthen and Miller, ' Geol. Surv. Illinois,' vol. vii, p. 331. 

 1883, ibid., p. 332, pi. xxxi, figs. 5 a— d. 



