10 



BRITISH FOSSILS. 



rows, the central space wide, granulated, and tuberculated, becoming bare and channelled 

 above ; spines slender (less than a line wide), solid, finely striated. A still larger 

 specimen, in M. Bowerbank's cabinet, measures 2^ inches across. 



Locality. — Gkey Chalk, Dover. (Coll. S. J. Mackie, Esq., and Bowerbank). 



23. D. submcdum, Ag., Cat. Echin,, p. 46. (Forbes.) 



Sub-pentangular, depressed; lat. 10^, alt. 4 lines ; base flat; apical opening pentangular, 

 its diameter 4 lines ; avabulacra straight, widening and confluent above ; the pairs of pores in 

 a compact double series as far as the fourteenth, a few next the mouth arranged in threes ; 

 primary tubercles small, subequal, those of the ambulacra in 2 close rows of 15 or 16 

 each, diminishing gradually from the circumference, and becoming very minute upwards ; 

 interambulacral tubercles in 2 principal rows of 11 or 12 each, diverging upwards, sur - 

 rounded by rows of granules below, and divided by a bare space above ; outer rows 

 shorter, of 6 or 7 tubercles each. 



Locality. — This species, which closely resembles the Oolitic D. suhangulare, Ag., 

 is one of the less common forms met with in the Upper Green Sand of War- 

 minster ; there are two specimens in the collection of Dr. Wm. Smith (Brit. 

 Mus.), and several in the Museum of the Geol. Survey. It is more abundant, and 

 attains a larger size in the chloritic marl of Somersetshire ; specimens in Dr. 

 Wright's collection measure 1 3 lines in diameter, and have 6 rows of interambulacral 

 tubercles, nearly equal in size at the circumference, like Diplopodia Roissyi, Desor. 

 In the collection of Dr. Wright is a specimen of this urchin from the U. Green 

 Sand of France, labelled "Z>. variolare, Br.," by M. Michelin. 



23 a. D. dubium, Albin Gras, Oursins Foss. Suppl., p. 3. f. 21 — 23. (Forbes). 



The specimens from Farringdon agree in their general character with a cast of the 

 typical D. dubium, in Dr. Wright's cabinet, and may be identified with D. sub- 

 nudum, Ag., to which species M. Desor unites the D. dubium, but their state of pre- 

 servation does not admit of very exact comparison. 

 Locality. — Lower Green Sand of Farringdon, Berks. (Brit. Mus.) 



24. D. Mackesoni, Forbes, 1. c. 



Sub-pentagonal, depressed, inflated at the angles ; lat. 18, alt. 7 lines ; apical opening 

 pentagonal, lat 7 lines ; ambulacra straight and rather wide, scarcely meeting above ; 

 pores obliquely two-ranked as far as the fourteenth paii- ; ambulacral tubercles divided by 

 2 rows of granules, and not diminishing much upwards ; interambulacral tubercles in 

 4 rows, the principal rows parallel, the outer rows shorter by 3 tubercles above. 



Locality. — From the Lower Green Sand (Kentish rag) of Hythe, presented to the 

 Museum of the Geol. Survey by Mr. H. B, Mackeson, Copt Pt. Folkestone. (Coll. 

 Mackie). It is nearly related to Z>, snbnudnm, with which, as the better preserved 

 species, it has been compared. 



25. D. Ibhetsoni, Forbes, Morris's Cat, p. 7 6. 



Circular, depressed, lat. 9, alt. 3^ lines ; apical opening 3, oral 3^- lines across ; base 

 tumid ; primary tubercles rather large and close-ranked, ambulacral 11-12, the 3 upper- 

 most minute ; interambulacral tubercles in two principal rows of 11 and 12 each, and two 

 external rows, shorter by 3 or 4 tubercles above, and diminishing rapidly beneath ; pores 

 obliquely two-ranked above, as far as the ninth pair, and again near the mouth. 



This species attains a larger size, but the examples hitherto obtained are fragmentary 

 and obscure ; the small specimen described is very distinct from Dr. Wright's 

 D. Autissiodoi^ensey which has only two rows of interambulacral tubercles, and wide 

 miliarj' zones. It is more; like D. Mackesoni, but is more closely tuberculated 

 above. 



Locality, — Lower Geeen Sand, Atherfield, Isle of Wight, (Mus. Geol. Soc.) 



