BRITXSH FOSSILS, 



3 



standing by itself. Each pair is distinctly surrounded by an oval 

 ring, which disappears when the surface is abraded. 



The mouth is small, and does not occupy more than a third of 

 the diameter, even including the short notches. These are ten in 

 number, and not far from equidistant ; the pair beneath the am- 

 bulacral spaces, however, are placed closer to each other than to the 

 next pair: the oral margin between the notches is straight, not 

 rounded. 



The apical disk is often well preserved, probably from its shape. 

 It is small, of five nearly equal heptagonal plates, the blunt points 

 of which form re-entering angles upon the interambulacral space, 

 and the madreporic plate is scarcely at all larger than the others. 

 Between these are five moderately large oculars of pentagonal form, 

 and these, as well as the genital plates, have the perforation placed 

 outside the middle of the plate. The anal aperture is scarcely 

 wider than one of the genital plates ; it is more angular than in our 

 figures. 



Affinities. — Between our British fossil and Agassiz's description 

 and figure of the P. rotata there seems to be a very close similarity. 

 Perhaps the figure in the Ech. Suisses may represent a species with 

 more numerous interambulacral plates on the upper half, but it 

 agrees with ours in the tumid form, the shape of the ambulacral 

 arese, and their close-set and equal tubercles. The latter character 

 we think will justify the union of P. ornata, Ag., with it, but will 

 at once separate P. rotata from the P. suhlcevis or P. aspera, Ag., 

 in which only every third or fourth of these tubercles is enlarged, 

 so that there is a zig-zag row of conspicuous tubercles down the area, 

 instead of the close-set double series of small ones. Added to this, 

 the two Swiss species just quoted are decidedly more depressed than 

 P. rotata, and the secondary tubercles, instead of being scattered, 

 are arranged in intermediate rows on the interambulacra. 



History. — M. Agassiz quotes in his Catal. Kais. the name of 

 Diadema micrococcon of Desmoulins for this species. As that 

 author published his Tableau Synonymique des Echinides in 1835,* 

 this name would have priority, had his short description, unaccom- 

 panied by figure or locality, been sufficient to recognize the species. 

 It was well figured, but scarcely sufficiently described in the Ech. 

 Suisses, where the more equal character of the ambulacral tubercles 

 and the greater height of the test are shown in contrast with the 

 depressed form of P. suhlcevis, Ag. But in the Catalogue Rais. 

 * Actes de la Soc. Liim, de Bordeaux, torn, 7. 



