2 



BRITISH FOSSILS. 



Tlie ambulacra are equidistant and of equal width, scarcely one- 

 fourth as wide as the spaces between them. The three anterior 

 ones are nearly direct, while the two posterior ones are a little 

 bent out near their origin, to conform to the tumid edge of the 

 large opening ; in the ordinary varieties they are not at all elevated 

 above the intervening spaces. They are composed of very numerous 

 (about 150) pairs of plates, judging from the number of primary 

 tubercles — one to about three plates — and from the pairs of pores. 



The primary tubercles in the ambulacral area are in four* rows 

 on the under surface and about halfway up the superior face, but 

 only the outer row^s continue to the vertex. These rows are placed 

 along the margin close to the pores, and there are about fifty-six 

 tubercles in each of them, of which thirty-six belong to the upper, 

 and twenty -two to the under surface. They are small, perforate, 

 and set upon but little elevated mamillse, which are sometimes 

 irregular or even slightly crenulate above, but often appear quite 

 smooth under the lens. 



The surface of the ambulacral and interambulacral spaces are 

 occupied by small granules not very closely placed, and mixed with 

 a few secondary tubercles, the areoke of which are distinctly sunk. 

 A single row of these granules separates each primary tubercle 

 ■from its neighbour, and they form rather a loose circle round each 

 areola. 



The avenues are not sunk in any of the typical specimens, and 

 the pores are ranked in single file, as usual in the genus. They 

 are rather large, and each pair (corresponding to one of the three 

 plates opposite each primary tubercle) consists of an inner roundish 

 and an outer oval pore, the tw^o being placed in somewhat opposite 

 directions, and separated by a strong elevated tubercle. 



The interambulacra have numerous rows of equal- sized tubercles 

 similar to those of the ambulacra, with an areola distinctly de- 

 pressed at the margin; they are larger, at least in the principal 

 rovvS, than those of the ambulacra, and more distant, there not 

 being above nineteen or tv^enty in a complete row on the upper 

 surface, and fourteen or fifteen below. They are arranged in the 

 following manner : tvv^o complete rows down the centre of the 

 interambulacral plates extend from the vertex to the mouth, and on 

 the inner side of these — first one, then two (see 4a), and near the 

 margin even four, rows appear placed in regular transverse rows on 

 the plates. Outside the principal rows an equal or gi'eater number 



* A fifth and, rarely, even a sixth, is interposed in some specimens at the actual margin, 

 hut not continued much ahove or helow it. 



