Dr. T. Wright on the Cassidulidse of the Oolites. 99 



set perforated tubercles, raised on eminences with crenulated 

 summits ; the pores are disposed in pairs in a single file. This 

 form is at present known only in a fossil state, and appertains to 

 the Oolitic period. 



Hyboclypus agariciformis, Forbes, n. sp. 



Test disciform, subpentagonal, and much depressed ; ambulacra 

 narrow, the three anterior straight, the posterior pair sinuous ; 

 interambulacra unequal and covered with an immense profu- 

 sion of microscopic tubercles ; apical disc central and vertical ; 

 anal valley deep with parallel sides, which gradually expand 

 about the middle of the single interambulacrum ; mouth-open- 

 ing small. 



Height ^ths of an inch, antero-posterior and transverse dia- 

 meters 3 inches and yoth. One large specimen measures, in the 

 antero-posterior and transverse diameters, 3 inches and T 6 ^ths. 



Description. — The outline of this Urchin varies a little in dif- 

 ferent individuals; in general the antero-posterior equals the 

 transverse diameter, but sometimes the transverse exceeds the 

 antero-posterior diameter. The ambulacra are of unequal width ; 

 the single anterior area is the narrowest, and the posterior pair 

 are the widest ; each pair of ambulacral plates carry from four to 

 six tubercles, which are so disposed that they form oblique rows 

 of from four to six in a row, which meet in the median line and 

 branch upwards and outwards, forming thereby V-shaped figures. 

 The pores are arranged closely together in pairs on the dorsal 

 surface, but from the basal angle to the mouth they are set wider 

 apart, and fall into triple oblique pairs. The interambulacral 

 arese are of unequal width j the anterior pair measure at the cir- 

 cumference 1 inch and T %ths, the posterior pair 1 inch and ^ths ; 

 the single area is yoth of an inch wider than the posterior pair. 

 The inequality in the width of these area? causes the subpenta- 

 gonal form of the test ; the interambulacral plates are slightly 

 bent upwards at the middle, and their surface is covered with 

 numerous small tubercles; in an ordinary-sized plate, I have 

 counted 100 tubercles, each of which is perforated and sur- 

 rounded by an areola. The base is flat and slightly undulated, 

 in consequence of the ambulacra forming straight valleys, and 

 the interambulacra convex conical lobes between the basal angle 

 and the mouth. The oral opening is situated nearer the ante- 

 rior than the posterior border ; it is small, of a pentagonal form, 

 and has the border surrounded by five small lobes formed 

 by the termination of the interambulacral arcae ; the tubercles 

 are large and more fully developed on the basal than on the 

 dorsal surface of the test, and the areola? are seen in this region 



7* 



