192 MAGNOTIA. 



entire surface of the plates of all the areas is covered with small, smooth, polished, equal- 

 sized tubercles, crowded closely together, and disposed in oblique lines (fig. 6/). 



The ambulacral areas are narrow and prominent, and nearly of the same width 

 throughout ; they are furnished with four rows of tubercles, except at the apex of the area, 

 where there are only two rows (fig. 6/). 



The poriferous zones form narrow, depressed, well-defined lines on the surface of the 

 test (fig. 6 d); the pores are small, and strictly unigeminal throughout, and there are four 

 pairs of pores opposite each inter-ambulacral plate (fig. 6/). 



The inter-ambulacral areas are upwards of four times the width of the ambulacral ; 

 each of these spaces is divided into two convex lobes by a well-defined longitudinal 

 depression, which extends from the base to the disc in the direction of the sutural line \ 

 the narrow ambulacra, bounded by the deep poriferous zones, and the wide inter- 

 ambulacra, divided by median depressions, produce a remarkable lobed appearance on the 

 test of this little urchin (fig. 6 a, b) ; the surface of the areas is crowded with small, 

 smooth, equal-sized tubercles ; at the circumference there are from twenty-five to thirty 

 rows (fig. 6/), but the number diminishes at the upper surface, where the areas contract, 

 and likewise at the base, where the tubercles are larger; it may be stated, as a general 

 character of this species, that the tubercles are crowded so close together in all the areas 

 that the surface of the plates is rendered invisible, and the test, when examined with a lens, 

 has a uniform granulated appearance (fig. 6 c, d). 



The base is concave (fig. 6 c), and the tubercles are larger in this region ; the mouth 

 opening is wide, and lies in a depression ; the peristome has a pentagonal form, from the 

 unequal size of the lobes. The notches are not well exposed in the only good specimen I 

 possess. 



The apical disc is small and prominent (fig. 6 a, b) ; it has a ring-like shape, from the 

 smallness of the ovarial plates and the size and position of the oculars (fig. 6 e) ; the 

 anterior pair of ovarials are the largest, and the right plate carries a small spongy madre- 

 poriform body; the vent is transversely oblong (fig. 6 e), and the oviductal holes are 

 large. 



Affinities and differences. — This urchin, at first sight, might be mistaken for Polycyphus 

 Normannus, Desor, from the Great Oolite, but it is more depressed and pentagonal, has a 

 more concave base, deeper areal depressions, smaller, more numerous, and closer-crowded 

 tubercles; the ambulacra are narrower, and the pores in the zones are unigeminal. These 

 characters are sufficiently marked to prevent Magnotia Forbesii being mistaken for 

 Polycyphus Normannus : Magnotia has the zones narrow and the pores unigeminal, Poly- 

 cyphus has the zones wide and the pores trigeminal. 



Locality and Stratigraphical position. — This very rare urchin was collected in the 

 upper ragstones of the Inferior Oolite, at Dundry, near Bristol. The two specimens in 



