280 PYGASTER. 



the base is slightly concave, and that the tubercles in that region are larger than those 

 on the upper surface (figs. 2 3 and/). 



Affinities and differences. — This species resembles P, semisulcatus in its pentagonal 

 form, but it is pyramidal and pentahedral, and is neither hemispherical nor depressed ; it is 

 distinguished from that species by a greater prominence of the ambulacra, the smallness 

 of the tubercles, the superficiality of the areolas, the microscopic character of the granules, 

 and the smallness of the vent. It is a very rare species, as I have only seen a specimen 

 in Mr. Lycett's cabinet, besides the one now figured in detail. 



Locality and Stratigraphical position. — I collected this urchin from the Pea-grit 

 at Crickley Hill. Mr. Lycett's specimen was found in the lower beds of the Inferior 

 Oolite, near Stroud. 



B. Species from the Cornbrash-= W Mage Bathonien, d'Orbigny. 



Pygaster Morrisii, Wright. PL XX, fig. 1 a, b, c, d, e,f. 



Pygastek Morrisii. Wright, Annals and Magazine of Natural History, 2d ser., vol. ix, 

 p. 92, pi. 4, fig. 1. 



— — Forbes, in Morris's Catalogue of British Fossils, 2d edit., p. 88. 



— — Wright, Memoirs of the Geological Survey, Decade V. Notes on 



British species of Pygasters. 



— — Desor, Synopsis des Echinides Fossiles, p. 166. 



Test pentagonal, depressed ; marginal fold very tumid ; single inter-ambulacrum 

 much truncated; ambulacral areas wide, convex, and prominent, with six rows of 

 tubercles; inter-ambulacral areas wide, with rather large tubercles, in very regular 

 vertical and horizontal rows, from twenty to twenty-two in each space on the same 

 horizontal line at the equator ; base flat, concave towards the mouth opening, which is 

 small ; anal opening long, pyramidal, occupying two thirds of the upper surface of the 

 inter-ambulacrum. 



Dimensions. — Height, eight tenths of an inch ; antero-posterior diameter, two inches 

 and three twentieths of an inch ; transverse diameter, two inches and a quarter of 

 an inch. 



Description. — This is one of our rarest Pggasters, and the specimen figured is the 

 only one I know. It has a thick test, with a very pentagonal outhne, is much depressed 

 on the dorsal surface, has a flat base, tumid sides, is remarkable for the size of its tubercles, 

 and for the regularity of their arrangement in vertical and horizontal rows. 



