284 ECHINOSPATAGUS 



The antero-lateral ambulacra are subpetaloidal, unequal, and lodged in considerable 

 depressions of the test. The anterior pair are the largest, and slightly flexed. The 

 poriferous zones are unequal in width, the inner being the narrowest. The external 

 zone is nearly as wide as the interporiferous space. The pores in both rows are nearly 

 equal in size, and are each connected with slitlike depressions of the test. The posterior 

 pair are shorter than the anterior. They are nearly as wide and diverge at an angle of 45° ; 

 their poriferous zones are nearly equal, and a considerable horizontal ridge separates the 

 pair. The plates of the test are sparsely provided with small perforated tubercles, which 

 are all raised upon crenulated bosses and surrounded by circular areolas, and the inter- 

 tubercular surface is covered with a fine close-set microscopical granulation. 



The apical disc is excentral, placed nearer the posterior than the anterior border, in a 

 depression at the ambulacral summit ; from this point the test developes a ridge which 

 extends to the posterior border, and the highest point of the test is found one quarter of 

 an inch behind the apical disc. 



The posterior border is very much elevated ; it appears to have a quadrate form, is 

 sharply truncated, and almost vertical. Tiiis portion of the test is unfortunately broken, 

 and it is from the remaining outline that the diagnosis is made. The vent was 

 placed high up on the border, but only one side of the periprocte remains to indicate the 

 position of that opening. 



The base is flattened in the region of the plastron ; it is slightly convex, and near the 

 oral region is concave. The raouth-opening, situated at the junction of the anterior 

 with the middle third, is transversely oval or subpentagonal ; the tubercles on the base 

 are larger than those on the upper surface. On the plastron they radiate from a central 

 point near the posterior border, and have a very symmetrical arrangement on the anterior 

 half; around the mouth they are larger and more sparsely distributed, whilst around 

 the anterior border and the ambitus they are again more numerous. 



Affinities and Differences. — I have grave doubts about this Urchin being a British 

 fossil. The matrix differs froio ^he rock in Wiltshire, which usually yields the Upper 

 Greensand fossils of that county, and therefore I record it with reservation ; it certainly 

 is the E. CoUer/nii of Sismonda so well figured by Desor in plate 40, figs. 2 to 4, of his 

 admirable Synopsis. Unfortunately I have no authentic specimen of this species with 

 which to compare it, although I have no doubt as to its identity with the form referred to. 



Stratic/raphical Position. — Said to have been collected from the Upper Greensand of 

 Wiltshire. I have no confirmatory evidence of the fact, and give it with proper 

 reservation. 



