\ 



6 BRITISH FOSSILS. 



of pores forming each ambulacrum varying slightly in width in different 

 specimens ; those of the two posterior ambulacra widest. The upper 

 part of the latter run close in contact and partly over the edge of the 

 superior or narrow portion of the anal groove. The furrow uniting the 

 pairs of pores varies in depth in different examples. The number of 

 conspicuous pores in each row dorsally is from 36 to 40. The spaces 

 separating the grooves linking the pairs of pores are minutely granu- 

 lated. The whole surface of the dorsal, lateral, and inferior ambulacral 

 and interambulacral plates is covered with thickly-set minute spiniferous 

 tubercles, lodged in moniliform depressions, the interspaces minutely 

 granulated. The ovarian plates are well covered with similar granules, 

 no tubercles. The ovarian pores are conspicuous, except the posterior 

 one, which is obsolete. Very distinct holes for the ocelli are seen at the 

 summits of the ambulacral avenues. The ventral surface is more or 

 less concave, most so in the pyramidal varieties. The ambulacra radiate 

 in depressed furrows from the mouth, becoming shallower towards the 

 margin. The mouth is more eccentric, and nearer to the anterior 

 margin than the apex. The tubercles on the ventral surface are rather 

 more scattered than on the dorsal. 



Comparative Dimensions of average Specimens of each of the three varieties 

 (all of which, however, insensibly pass into each other), iyi inches and 

 twelfths. 





a. 



|S 



y 





If. 



01^ 



It'. 





If. 



0\\ 







Of. 



Of. 



Of. 





Of. 



Of. 



Of. 



Mouth to anterior margin .... 



Of. 



Of. 



^ 1. 





Of. 



0,^ 



0?. 



Breadth of anal furrow at anus , 



Ofl 



Of. 



Of. 



British Localities and Geological Range. — Inferior Oolite, Stroud 

 (B. M.). Great Oolite, Minchinhampton (B.M.). Cirencester (S. 

 P. Woodward). Cornbrash, Yorkshire (Phillips). Abundant in corn- 

 brash of Dorset (Bristow, Gapper). Chippenham, small gray variety 

 (S. P. Woodward). Northampton (B. M.). 



Foreign Distribution. — (Extracted from the catalogue of Agassiz and 

 Desor.) " Oxford. d'Alencon, Courgains (Sarthe), Calc. a Polypiers 

 de Ranville, Coulie. — var. minor. Forest Marble de Chatelcensoir." 

 Form latiporus, " Cornbrash -de Metingen, Maiche." Form gracilis, 

 " Ool. Ferrug. de Durrenast (Jura Soleurois)." 



