92 Dr. T. Wright on the Paleontology of the Isle of Wight. 



structure are very distinct from that form. The circumference 

 is pentagonal from the convexity of the ambulacral arese, and the 

 base and summit are much depressed. 



The interambulacral areas are one-third broader than the 

 ambulacral ; two rows of primary tubercles occupy the centre of 

 the plates ; there are about ten pairs of tubercles in each area, 

 which are of a moderate magnitude, and gradually diminish in 

 size from the circumference to the base and summit ; the mani- 

 millary eminences are small, their summits are deeply crenulated, 

 and the tubercles, of proportional size, are deeply perforated; at the 

 circumference six rows of granules separate the tubercles from each 

 other, but towards the upper surface the four central rows are 

 absent, which leaves a naked space in the middle of the area ; 

 three rows of granules in like manner separate the tubercles 

 from the poriferous valleys ; at the base of the area, and extend- 

 ing as far as the circumference of the test, are incomplete rows 

 of secondary tubercles ; these gradually diminish in size and dis- 

 appear at the upper surface, which is occupied with an unequal- 

 sized, close-set granulation about three rows deep ; the ambu- 

 lacral areas are one-third narrower than the interambulacral, 

 they are however very prominent and convex, and are occupied 

 with two rows of primary tubercles about ten in a row ; the lower 

 six pairs of tubercles are nearly as large as the corresponding 

 tubercles in the interambulacral areas, but the upper four pairs 

 are much smaller, so that whilst there is a gi'eat uniformity in 

 the size and form of the tubercles on the base and circumference 

 of the test, there is a very marked diiference between those of 

 the ambulacra and those of the interambulacra in the vicinity of 

 the ovarial disc; the intertubercular space is occupied by a zig- 

 zag band of granulation, which is narrow below where the tu-l 

 bercles are large, but becomes broader above where they are! 

 small. The pores are arranged in double pairs near the summit 

 and mouth, but in' the other part of the avenues they are in^ 

 single pairs ; the apical disc is absent in our specimen, and the| 

 mouth is large and decagonal. 



Affinities and differences. — This species nearly resembles Z>.| 

 Bourgueti, Ag., but differs from it in the rudimentary condition 

 of the upper tubercles of the ambulacra, and in having the pores 

 in double pairs above and below ; the intermediate granulation is 

 likewise less homogeneous than in D. Bourgueti. 



Locality. — I collected this Urchin from the lower greensand at 

 Atherfield, in No. 4 of the Cracker group of Dr. Fitton^s section : 

 it must be very rare, as none of the cabinets of Atherfield fossils 

 hitherto examined by me contain a specimen of this Diadema. 

 It has been collected by M. Cottcau from the Neocomicn stage 

 at Auxerre, where it is likewise very rare. 's 



