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PYGURUS 



Test large, sub-circular, sometimes oval, and slightly rostrated posteriorly ; upper sur- 

 face flattened, and much depressed ; base sub-concave, rounded anteriorly, and slightly 

 produced posteriorly ; ambulacral areas on the upper surface nearly equal-sized and lan- 

 ceolate j poriferous zones petalloid near to the border ; apical disc small, nearly central ; 

 inter-ambulacral areas broad and flat, with a very distinct zigzag median suture ; margin 

 very thin j base sub-concave ; mouth-opening small, situated nearer the anterior than the 

 posterior border. 



Dimensions. — a. Antero-posterior diameter, six inches and four tenths ; transverse 

 diameter, six inches ; height indeterminable. 



b. Antero-posterior diameter, five inches and one fourth ; transverse diameter, four 

 inches and nine tenths ; height, one inch and three tenths. 



Description. — This large discoidal urchin is remarkable for the great size it attains ; 

 nearly all the specimens I have seen are broken, and more or less imperfect, so that the 

 identification of the species is extremely difficult. Last summer, however, I met with 

 one which had retained the form of its circumference, as well as the shape of its upper 

 surface, and this example enabled me to identify the species I had formerly named Pygurus 

 giganteus with Koch and Dunker's Clypeaster Hausmanni. It is, therefore, extremely 

 interesting to find this urchin in the same horizon of the Coralline Oolite of Malton, 

 the zone of Cidaris Blumenbachii, the one it occupies in the Korallenkalk of northern 

 Germany. 



Pygurus Hausmanni has in general a sub-circular outline, rather inclining to an oval, 

 its transverse diameter being always less than its antero-posterior measurement ; the 

 anterior border is rounded, and in specimen b the posterior border is a little produced ; 

 the upper surface is moderately convex in the smaller specimen, but is very much flattened 

 in the larger ones, and the anterior half is more convex than the posterior half. 



The ambulacral areas are narrow and lanceolate ; they have six rows of small tubercles 

 in their widest part, which are not all arranged in a horizontal series on the two corres- 

 ponding plates of the area, but are disposed thereon so as to form oblique V-shaped rows. 

 Plate XXXIX, fig. 2, exhibits this arrangement of the tubercles. The poriferous zones are 

 moderately wide, the holes of the inner row are round, those of the outer row are slit-like, 

 of which there are eight pairs opposite each large plate (fig. 2) ; the septum between each 

 pair of holes supports on its upper surface a horizontal row of nine small granules. The 

 ambulacral areas and poriferous zones form together a series of five elegant leaf -shaped 

 petals, which are enlarged in the middle, become lanceolate near the disc, and are con- 

 tracted at the circumference ; the poriferous zones are petaloidal six sevenths of their 

 length ; and near the lower seventh the pores approximate ; in their course round the 

 margin, and across the base they remain close together in pairs. 



