BRITISH FOSSILS. 



3 



nal. The anus is elliptical, one-third greater in vertical dimensions 

 than the mouth, and is placed at the hinder extremity in the lower 

 part of the posterior interambulacral area immediately above the 

 margin. It varies a little in degree of elevation in different speci- 

 mens. Its margins are occasionally, though rarely, slightly thickened. 

 The plates, both dorsal and ventral, are thickly covered with miliary 

 granules, among which are distributed the spiniferous tubercles, per- 

 forated and placed on a crenulated protuberance within an areola. 

 These tubercles are very little smaller on the dorsal surface than 

 they are on the ventral. On the centro-lateral interambulacral plates 

 there are from 15 to 30 tubercles. On the interambulacral areas they 

 fall into obscure oblique ranks of three or four. In a specimen six- 

 tenths of an inch in height, there are about 22 rows of plates to each 

 half of an interambulacral area, and about six ambulacral plates with 

 as many pairs of pores to each of the former. 



The apical disk is composed of five genital and five ocular plates ; 

 four of the former are perforated, and one, the posterior one, imper- 

 forated and small. The left anterior genital plate is larger than the 

 others, and produced to form the central madreporiform plate. The 

 remainder of the disk is granulated with scattered miliary tubercles. 

 All the ocular plates are perforated. The pairs of pores are in single 

 file on the back and sides ; but when they turn the margin to proceed 

 down the under surface to the mouth, they gradually fall into ranks of 

 three pair, always very oblique, and never very closely set. The spines 

 are unknown. Internal casts exhibit the ambulacra in prominent 

 relief. 



The larger variety is found at Warminster, and is more depressed 

 usually than the smaller. In no essential character do they differ 

 from each other. 



The following measurements of six specimens will show the variability 

 of proportions : — 









2(/3) 



3(/3) 







6(^) 







Length . . 



111 



Oil 



Oil 



1 



Oil 



OA 







Breadth . . 



h\ 



Of, 



Oji 



Oji 



010 



Ot^ 







Height . . 



0^ 



0^2 



Oil 



0^ 



Ot^ 



0^ 





Localities and Geological Position. — This Galerites is characteristic 

 of the bed of chalk marl, tinged green, with scattered particles of sili- 

 cate of iron, which occurs, occupying a thickness frequently of two or 

 three feet, at the junction of the chalk marl and green sand, and which 

 has been described by Captain Ibbetson under the name of Chloritic 



