4 



BRITISH FOSSILS. 



ambulacral plates, and its prominent sides to the avenues of pores. The 

 ten deep notches are caused by as many internal ribs, which spring 

 from the inner sides of the mouth, and run up the wall under the cari- 

 nated portion of each series of interambulacral plates. 



Dimensions and Varieties. —The ordinary breadth of this species is 

 about 7-12ths of an inch. The proportions of the breadth to the height 

 are very variable, even in specimens from the same locality, as may be 

 exemplified by the following instances of upper green-sand specimens 

 from AVarminster : 1st specimen, breadth to height (in millemetres,) 

 14 : 10 ; 2nd, 14 : 8 ; 3rd, 12:8; 4th, 11:9; 5th, 10 : 8. This 

 variation in degree of elevation is accompanied by very great variation 

 in rotundity of margin and convexity of base. The most rounded spe- 

 cimens are those of a variety which occur in the white chalk. It com- 

 bines rotundity with height, and has usually fewer primary tubercles 

 than are seen in ordinary specimens. In no essential respect, however, 

 does it differ from the green-sand form, and may be paralleled exactly 

 by exceptional specimens of the latter. 



In the " Monograph of Galerites," by Desor, and in the " Catalogue 

 Raisonne des Echinides," four species of Discoidea allied to suhuculus, 

 are enumerated. The first of these, Discoidea minima (Agassiz), 

 founded on a single example from the chalk marl of France, seems, 

 judging from figure and description, to be only one of the less conical 

 forms of the young of the species, such as not uncommonly occur at 

 Warminster. The second, Discoidea pisum (Merian), unfigured, is said 

 to be exactly like D. minima, and only distinguishable from it and 

 suhuculus by having distinctly perforate primary tubercles. This, how- 

 ever, is the case with well-preserved specimens of every Discoidea, and 

 is a generic, not specific, distinction. The third, Discoidea turrita 

 (Desor), described and figured from a single example, is represented 

 as proportionally higher than suhuculus, and differing in having several 

 vertical series of primary tubercles equally prominent on the inter- 

 ambulacral spaces, and not two only, like those on the keels. But we 

 find in British specimens these characters combined or not, and occur- 

 ring in individuals not uncommonly, besides exhibiting every degree of 

 transition into the ordinary features of the species. The fourth is 

 merely named in the " Catalogue Baisonne ;" it is the Discoidea infer a 

 (Desor), from the white chalk of Fecamp, said to be very near 

 suhuculus, but differing in having the primary tubercles conspicuous only 

 on the ventral surface. Specimens of our variety from the white chalk 

 — the most distinct looking form of all — yet certainly not essentially so, 

 accord very well with the brief and insufficient notice given in the cata- 

 logue. 



Allied Species. — Although I have little doubt that the several so- 



