314 Dr. T. Wright on the Cassidulidae of the Oolites. 



convex, and nearly equal ; apices closely approximated ; inter- 

 ambulacral arese of unequal width, base much undulated; 

 mouth nearly excentral ; anal opening oval, inframarginal, and 

 situated in a deep depression with inclining walls. 



Height 1 inch and y^ths, antero-posterior diameter 3 inches 

 and T 3 oths, transverse diameter 3 inches and yo^h. 



Description. — This Urchin was first noticed by Mr. Young in 

 the Coralline Oolites of Yorkshire. It has an ovoidal or subpen- 

 tagonal form and is much depressed, the vertex is nearly central, 

 and from the circumference of the small disc the broad petaloid 

 ambulacra diverge. The anterior single ambulacrum is the 

 narrowest, the posterior pair the widest, and the anterior pair 

 of intermediate width, but the difference between the five am- 

 bulacra is very trifling ; they have all a petaloid form and their 

 apices are blunt and much approximated. 



The space between the internal rows of pores is proportionally 

 broad, and the pores are distant and connected by distinct sulci ; 

 about the inferior third of the ambulacra the pores approxi- 

 mate, from whence they pass in close-set pairs to the border of 

 the* test. The leaf-like form, therefore, which the ambulacra 

 present is in a great measure owing to the disposition of the 

 pores and their proximity or remoteness at different parts of their 

 course. The interambulacra are of unequal width ; the anterior 

 pair are the narrowest, and the posterior pair and single area are 

 alike in width ; they are uniformly convex above. The single 

 area is produced and deflected, and in this region the anus is 

 situated ; the apical disc, which occupies the centre of the vertex, 

 is very small, depressed, and composed of four small perforated 

 ovarial plates, and a single imperforate plate with a small central 

 spongy body as in Nucleolites. The base is flat, somewhat con- 

 cave, and much undulated in consequence of the ambulacra 

 forming straight valleys and the interambulacra intermediate 

 convexities. The anterior border is truncated, and as the left 

 interambulacrum is not so much developed as the right, there 

 is a slight inequality of the anterior border, which may however 

 be only an abnormal variety ; the anus occupies the extreme por- 

 tion of the single interambulacrum, it is of an oval form, the long 

 diameter corresponding to the antero-posterior diameter of the 

 test, and is situated in a deep depression with highly inclined 

 sides. The mouth is slightly excentrical, but is partly concealed 

 by the matrix. 



Affinities and differences. — This species so very much resembles 

 Pygurus (Clypeaster) Hausmanni of Koch and Dunker, that we 

 have been almost disposed to consider the latter as a gigantic 

 example of this species. The dorsal surface of C. Hausmanni is 

 flatter and more convex, the poriferous sulci are shorter, and the 



