Dr. T. Wright on the Cidaridaj of the Oolites. 259 



small tubercles at the base, and six rudimentary tubercles at 



the apex of the area? ; mouth moderate and decagonal. 



Height /yths of an inch, transverse diameter f^ths of an inch. 



Description. — The spheroidal test of this Urchin is much de- 

 pressed at the anal pole and flattened at the base. The ambu- 

 lacral arese are nearly straight and of a tolerably uniform width 

 throughout, and furnished with two rows of small, quite micro- 

 scopic, but nevertheless mammillated and perforated tubercles, 

 about fourteen in each row, disposed alternately on the margins 

 of the arese, and increasing slightly in size towards the basal 

 angle. The base of the arese has four pairs of larger tubercles as 

 in the other species of this genus. The interambulacral arese are 

 nearly three times the width of the ambulacral, and furnished 

 with two rows of tubercles from 9-10 in each row, the three 

 pairs at the equator of the test alone attaining their full develop- 

 ment ; those at the base being of a secondary size, whilst those 

 on the upper part of the arese are disproportionately small and 

 even rudimentary. The upper surface of the test is covered with 

 small close-set granulations, in the midst of which the rudi- 

 mentary tubercles rise at distant intervals apart. The mammil- 

 lated eminences of the six large tubercles are surrounded by 

 well-defined areolse, which are confluent at their upper and lower 

 margins ; but down the centre of the arese two or four rows of 

 granules, and at the lateral borders thereof one or two rows of 

 granules descend, which form lateral wreaths surrounding the 

 side margins of the areolse : these marginal granules are larger 

 and more uniform in their arrangement than those occupying 

 other parts of the surface of the test. 



The mouth-opening, of a decagonal form, is one-half the dia- 

 meter of the body, with deep marginal notches dividing its 

 circumference into ten nearly equal lobes, those of the ambulacral 

 arese being the largest. 



The apical disc is either absent or concealed in the specimens 

 before me, and the spines are unknown. 



Affinities and differences. — H. confluens resembles H. Thur- 

 manni, Ag., in its depressed form and in the small number of the 

 primary tubercles on the interambulacral arese; it is distin- 

 guished from that species in the partial absence of the circle of 

 granules which entirely surround the tubercles in H. Thurmanni, 

 and in the rudimentary condition of those occupying the upper 

 surface of the test. The ambulacral arese are nearly straight in 

 H. confluens, and much undulated in H. Thurmanni. This Urchin 

 has many points of affinity with Acrosalenice, but our ignorance 

 of the apical disc leaves a doubt in our mind whether it may not 

 belong to that genus. Until specimens with the disc preserved 

 are found, that doubt cannot be removed. 



17* 



