FROM THE INFERIOR OOLITE. 175 



their apices form angles re-entering into the inter-ambulacral areas (fig. 1 b) ; the ocular 

 plates are small pentagonal pieces, firmly wedged between the genitals ; the eye-holes 

 are very minute, and the surface of the discal elements is covered with numerous small 

 granules ; the oviductal and eye-holes are perforated about the junction of the outer with 

 the middle third of the plates. The anal aperture is central and circular (fig. 1 d), and 

 in diameter is about the length of one of the genital plates. 



Affinities and differences. — This species very much resembles Pedina Gervillii, 

 Desmoulins, in fact, it requires a close and critical comparison of good specimens to discover 

 the differences between them ; Pedina rotata has more tumid sides, the ambulacral areas 

 possess a greater number of small, equal-sized tubercles, more closely set together, and 

 arranged with greater regularity than in Pedina Gervillii ; the poriferous zones are like- 

 wise wider, and the pores lie more oblique, with two granules between each rank ; the 

 primary tubercles are larger and more prominent ; and there is, consequently, a greater 

 disproportion between the tubercles of the ambulacral and those of the inter-ambulacral 

 areas than in Pedina Gervillii. 



It differs from Pedina sublevis in having larger tubercles in the inter-ambulacra, and 

 in the number, approximation, and regularity of the arrangement of the ambulacral 

 tubercles. 



M. Cotteau* has examined the types of M. Agassiz' species contained in the magni- 

 ficent collection of M. Michelin, and has given the following analysis of the affinities and 

 differences which exist between the four species he describes, and which I have translated 

 for this section. 



" 1st. The Pedina sublevis, Agassiz. which is characterised by its great height, its pores 

 disposed by very oblique triple pairs, its primary tubercles slightly developed, spaced out, 

 and confounded, so to speak, especially at the base, with the secondary tubercles which 

 accompany them. We consider the Pedina ornata, Ag., as a variety of this species. 



" 2d. The Pedina Gervillii, Desmoulins ; remarkable by its depressed form, its principal 

 ambulacral and inter-ambulacral tubercles few in number and spaced out, its secondary 

 tubercles very small, its pores largely open, ranged in triple oblique pairs, with little 

 obliquity, and, consequently, enclosed in narrow poriferous zones. 



" 3d. The Pedina rotata, Wright ; assuredly very near to Pedina Gervillii, but distin- 

 guished from it by its more tumid form, its pores disposed more obliquely, its ambulacral 

 tubercles smaller, closer set together, more numerous, and forming on the borders of the 

 poriferous zones perfectly regular ranges. This last character suffices to distinguish Pedina 

 rotata from young individuals of Pedina sublevis. We retain to this species the name 

 rotata, with this observation, that in the figures of the ' Echinodermes de la Suisse' 

 (pi. xv, figs. 4 — 6), the mouth is relatively smaller, and the ambulacra! tubercles are more 

 spaced out. 



* 'Etudes sur les Echinides Fossiles,' p. 315. 



