172 Dr. T. Wright on the Cidaridse of the Oolites. 



thickly studded with small smooth tubercles ; at the widest part 

 there are from twenty-five to thirty rows ; the number of these 

 diminish at the apex and base, the basal tubercles are however 

 larger than the others ; the ambulacral areas are narrow and of a 

 nearly uniform width ; they are furnished with four rows of small 

 tubercles similar to those occupying the interambulacral areas ; 

 they are in fact so closely set together that the plates are invi- 

 sible, so that the test presents only a uniform granulated surface ; 

 the avenues are straight, narrow, but well defined ; the pores are 

 closely arranged in simple pairs ; the base is concave and the 

 tubercles in this region are larger ; the mouth presents almost a 

 pentagonal form in consequence of the wide straight arch made 

 by the margin over the ambulacra and the small angles which 

 the shallow notches make in the interambulacra; the apical disc 

 is narrow and prominent ; the madreporiform is larger than the 

 pairs of ovarial plates, and the oculars are small, but apparently 

 soldered into the angles formed by the ovarials. 



Affinities and differences. — The greater number and the dimi- 

 nished size of the tubercles, with the deep median furrow down 

 the centre of the interambulacral areas, serve to distinguish A. 

 Forbesii from A. nodulosa : as they are the only two forms of this 

 genus hitherto found in our Oolites, these characters form a good 

 diagnosis. 



Locality and stratigraphical range. — This Arbacian was collected 

 from the upper beds of the Inferior Oolite near Dundry, and we 

 only know it from that locality. I have dedicated this species to 

 Prof. Edward Forbes, to whose genius, talents, and learning 

 natural history is so largely indebted. 



Arbacia nodulosa, Wright. PL VI. fig. 3 a, b. 



Syn. Echinus nodulosus, Goldfuss, Petr. Germanise, t. 40. fig. 16. 

 p. 125. 



Test hemispherical, with a subpentagonal circumference; ambu- 

 lacral areas prominent and bounded by deep poriferous avenues ; 

 interambulacral arese divided by a slight median depression ; 

 tubercles nearly equal-sized in both areas, and arranged in lon- 

 gitudinal rows. 



Height /(jths of an inch, transverse diameter |^ths of an inch. 



Description. — This nodulated Urchin is hemispherical and has a 

 subpentagonal form from the development of the ambulacral areas, 

 which are very prominent, especially at the basal angle ; they are 

 furnished with three rows of smooth prominent spherical tubercles 

 set at short distances apart, the central row being absent at the 

 bast- and apex of the area? ; the interambulacral areas are twice the 

 width of the ambulacral, and are occupied at their widest parts 



