186 Dr. T. Wright on the Cassidulidse of the Oolites. 



interambulacra with from 20 to 22 rows of tubercles at the 

 circumference ; base flat, depressed towards the eentre; the 

 single interambulacnun truncated posteriorly ; ami! opening 

 large, occupying three-fourths of its upper surface. 



Height T n a ths of an inch, antero-posterior diameter 2 inches 

 and /(jths, transverse diameter 2 inches and Aths. 



Description. — The ambulacral area? of this Urchin are wide, 

 convex, and prominent, and form an exception to Agassiz's 

 generalization, that in the genus Pygaster the ambulacra are 

 furnished witli only four rows of tubercles, for in this species 

 there are six well-developed rows at the circumference of the test 

 (fig. 1 c) ; the two marginal rows extend from the mouth to the 

 vertex, the two middle rows commence at a short distance from 

 the mouth and terminate at a short distance from the vertex, and 

 the two internal rows commence about four lines above and extend 

 about the same distance beyond the basal angle. The interam- 

 bulacral area? are three times the width of the ambulacral, and 

 are furnished with twenty-two rows of tubercles at the circum- 

 ference of the test ; they attain their greatest development at the 

 base of the area?, and are arranged so uniformly, and disposed so 

 closely together, that they present somewhat the appearance of a 

 mosaic work (fig. 1 d) ; the areola? are deeply sculptured out of 

 the plates, and surrounded by circles of very small granules ; of 

 the twenty-two rows of tubercles which occupy the area at the cir- 

 cumference, only six, and those the three central rows of each 

 column, extend from the mouth to the vertex, the others are limited 

 to shorter distances, the length of their range being in proportion 

 to their distance from the margin of the columns. The poriferous 

 avenues are narrow, and the pores are small and set closely together 

 in pairs. The basal angle is tumid (fig. I b) ; the base is flat and 

 depressed towards the centre; in this depression the mouth is 

 situated ; the opening is small, being only one-sixth the diameter 

 of the test at the circumference. The anus is a large oblong 

 opening (fig. 1 a), occupying at least the upper three-fourths of 

 the single interambulacrum. The apical disc is absent ; the spines 

 are short, needle-shaped, and finely striated longitudinally. The 

 test is very thick. 



Affinities and differences. — This species resembles P. laganoides, 

 Agassiz, in its depressed form, obtuse basal angle, and truncated 

 single interambulacrum ; but P. Morrisii is distinguished from P. 

 laganoides in possessing a greater number of tubercles in both 

 arese, P. laganoides having four rows in the ambulacral and 

 twelve in the interambulacral, whilst in P. Morrisii the corre- 

 sponding area? possess six and twenty-two. We know no other 

 species among its congeners for which it could be mistaken. 



