2 



BRITISH FOSSILS. 



joins the apical disk a little in front of the tips of the antero-lateral 

 ambulacra. These are placed very far forward, and run down 

 upon the most tumid portion of the body. In a large specimen 

 as many as thirty pairs of pores may be counted in these rows, 

 between the apex and the margin. The avenues are at the sur- 

 face of the test, and the pairs of pores of their outer rows are a 

 very little wider than the inner ones. The posterolateral am- 

 bulacra are even less conspicuous ; they diverge greatly from the 

 antero-laterals, and form together a very acute angle. Their upper- 

 most portion consists of closely set pairs of pores ; on their posterior 

 portions the pairs of pores become very indistinct. 



The plates on the margins of the anteal sulcus are thickly 

 studded with small primary tubercles, which are very numerous 

 also on the cheeks, but become few and scattered on the dorsal 

 surface. They are perforated, and placed on crenulated bosses. 

 The fasciole is distinctly marked, passing under the vent, and con- 

 tinued on each side even over the cheeks. The vent is large and 

 perpendicularly ovate, its widest diameter uppermost ; the area 

 in which it is placed is either quite plane or slightly excavated. 



On the under surface the mouth is seen near the anterior ex- 

 tremity, in front of the widest portion of the test ; it is compara- 

 tively small and transversely oblong, the hinder lip is highest. 

 On the ambulacral portions of the base there are very few or no 

 tubercles, but a considerable number, and those ra^ther large, on the 

 shield-shaped inferior portion of the odd interambulacrum. In all 

 the specimens that I have seen the base is much abraded. Young 

 examples are rather more elevated than old ones. In one of Mr. Rose's 

 specimens, however, a flint cast from Swafl'ham, -ffths of an inch 

 in length, nearly the same proportions are shown as in an example 

 of twice the size. The most perfect specimen, in point of com- 

 pleteness of outline, that I have seen, measui'es one inch and ten- 

 twelfths in length, by one inch and four-twelfths in breadth. 

 Its anterior extremity is one inch and three-twelfths, and its 

 posterior nine-twelfths of an inch in height. 



Locality and Geological Position' First observed by Mr. Rose 

 of Swafl'ham, and communicated by him to Mr. Samuel Woodward, 

 in whose Geology of Norfolk it was briefly noticed, and slightly but 

 characteristically figured. It is recorded by these geologists from the 

 Upper and Medial Chalk of Norfolk. Mr. Bowerbank possesses a fine 

 flint cast showing the plates. This description is drawn up chiefly 

 from Mr. Rose's original examples and Mr. Bowerbank's specimen, 

 all kindly lent for the purpose, and from a fine, but imperfect, 



