FROM THE GREAT OOLITE. 241 



Dimensions. — Height, seven twentieths of an inch ; transverse diameter, three quarters 

 of an inch. 



Description. — This small depressed urchin was kindly communicated to me by my friend 

 Mr. S. P. Woodward, and is the only specimen of the form which has come under my 

 notice. The test is circular, much depressed, and well characterised by the dispropor- 

 tionate magnitude of the equatorial tubercles, when compared with those on the upper and 

 under surfaces of the test. 



The ambulacral areas are narrow, and slightly undulated ; they have two rows of very 

 small tubercles on the margins, from twelve to fourteen in each row, placed wide apart 

 from each other, with a few granules in the interspace (fig. 4 c) ; the poriferous zones are 

 narrow and undulated, winding round the bulging areolas of the large equatorial tubercles; 

 there are from eight to nine pairs of pores opposite the large tubercular plates (fig. 4 c), 

 and the septa form mammillated elevations on the surface. 



The inter-ambulacral areas are more than four times as wide as the ambulacral (fig. 

 4 a) ; they have only six plates in each column (fig. 4 b), each plate bearing a primary 

 tubercle ; the two equatorial plates support tubercles with very large prominent bosses 

 (fig. 4 b), which are likewise surrounded by wide, smooth areolas ; on three parts of the 

 margin of these equatorial plates there is a continuous series of small granules, which are 

 absent, however, from the lower border (fig. 4 c) ; on the smaller of the larger plates the 

 bosses become suddenly less, and between the areola and the centro-suture there are three 

 rows of granules (fig. 4 c) ; the uppermost plate has a mere rudimentary tubercle on its 

 surface, surrounded with numerous granules (fig. 4 a) ; the basal tubercles likewise sud- 

 denly diminish in size towards the peristome ; the zones expand in consequence, and the 

 pairs of pores are disposed in oblique ranks of threes closely laid together. 



The apical disc is of moderate size for the genus Acrosalenia (fig. 4 a) ; the four ovarial 

 plates are shield-shaped, the anterior pair being a little larger than the posterior pair 

 (fig. 4c?); the single ovarial plate is small and crescentic, the oviductal holes are perforated 

 near their apices, and the small spongy madreporiform body occupies the centre of the right 

 anterior (fig. 4c?); the sur-anal plate is composed of three pieces, one central pentagonal 

 piece, and two small latero-posterior rhomboidal pieces, which form the anterior wall of the 

 vent (fig. 4c?); the sides of this opening are formed by the posterior genital and ocular 

 plates, and the posterior wall by the single crescentic genital (fig. 4c?); the three ante- 

 rior ocular plates are heart-shaped, the posterior pair are rhomboidal, and the eye-holes are 

 all marginal (fig. 4c/); all the plates are moderately large. 



The base is flat, the mouth opening large, and the peristome decagonal ; the notches 

 are deep and have reflected margins ; the opening is one half the diameter of the test, 

 and the ambulacral are larger than the inter-ambulacral lobes. 



Affinities and differences. — This urchin at first sight resembles some of the depressed 



