276 PYGASTER. 



When Professor Phillips figured Pygaster semisulcatus in his ' Geology of Yorkshire,' 

 he was not aware that two distinct species of the genus Pygaster existed in the Oolites of 

 that county, one collected from the Inferior Oolite of Whitwell, and the other from 

 the Coralline Oolite of Malton ; as the type specimen is now lost, it is uncertain whether 

 the Whitwell or Malton urchin was the one sketched in tab. 3, fig. 17 of that work. 

 When Professor Forbes was studying this species, he applied to Professor Phillips for the 

 type of P. semisulcatus, and the one sent was a Whitwell specimen, which I compared 

 with my Inferior Oolite urchins, and ascertained the identity of the species. 



It was assumed, therefore, by Professor Forbes, that the Whitwell urchin was 

 P. semisulcatus, and his beautiful plate of this species, in the fifth Decade of the 

 ' Memoirs of the Geological Survey/ was lettered accordingly. 



The general form is sub-hemispherical, more or less depressed on the upper surface ; 

 the base is flat or slightly concave, and the mouth is lodged in a considerable central 

 depression ; the circumference is sub-pentagonal, and the bilateral symmetry of the test is 

 very evident in specimens which are free from distortion (fig. 1 a) ; the upper surface is 

 in general convex (fig. 1 c), but sometimes in large examples the sides are flattened, and 

 the test then assumes a conoidal figure, when it becomes the P. brevifrons, M'Coy. 



The ambulacral areas are equidistant, and of equal width, and about one fourth 

 the breadth of the inter-ambulacral areas ; the anterior single area is straight and 

 lancet-shaped ; the anterior pair are gently bent backwards, and the posterior pair in 

 large individuals have a slight/-shaped flexure, occasioned by the great width of the vent 

 (fig. 1 a). They are composed, according to the size of the test, of from one hundred and 

 twenty to one hundred and sixty pairs of plates, as weathered specimens show that every 

 third plate carries a primary tubercle (fig. 1/). There are two complete rows of tubercles 

 on the margins of the areas, which extend without interruption from the peristome to the 

 disc ; in each row there are about fifty-eight tubercles ; of these, twenty-two belong to the 

 base and margin, and thirty-six to the sides and upper surface (fig. 1 a, b) ; within these, 

 two other rows commence about half an inch from the peristome, and extend from thence 

 three quarters of an inch up the sides, about which point they disappear ; at the mar- 

 ginal angle sometimes a fifth, or in some cases a sixth, row is introduced for a very 

 limited extent, and the miliary zone in the upper part of the area is filled with small granu- 

 lations. The poriferous zones are narrow ; the pores are strictly unigeminal throughout, 

 one pair of pores nearly corresponding to each of the ambulacral plates. The pores con- 

 stituting a pair are placed slightly oblique ; in some specimens the inner hole is round 

 and the outer is oval, but this character is individual, and not general. The septa are 

 narrow, and form very inconsiderable elevations on the surface (fig. If, h). 



The inter-ambulacral areas are four times the width of the ambulacral ; they are 

 composed of about thirty-five pairs of plates, of which about fifteen occupy the base, 

 and twenty the sides ; these plates, according to their breadth, support a variable number 

 of tubercles, those at the margin having from nine to ten tubercles on the same horizontal 



