2 



BKITISH FOSSILS. 



mistake.* And as Prof. E. Forbes lately published, as an Echinopsis, 

 a tertiary species agreeing in all the main characters with Pedina, 

 he has in that way distinctly recorded his opinion that the two 

 groups should form but one genus. 



Description. — Diameter, an inch and a quarter ; height, three- 

 quarters of an inch. " The test of this urchin is circular, but in some 

 specimens a fullness of the ambulacral arese gives it a slightly pen- 

 tagonal outline ; it is particularly tumid at the circumference, and 

 depressed at both poles. The ambulacral arese have two rows of 

 small tubercles disposed along their outer border, between which 

 rows small granules are arranged with less regularity. The inter- 

 ambulacral arese are twice and a half the width of the ambulacral, 

 and furnished each with a double range of primary tubercles, ex- 

 tending from the mouth to the ovarian plates.'' These primaries are 

 prominent, and give a rough character to the upper surface, where 

 they are larger and less crowded than on the lower. They radiate 

 in ten conspicuous rows, each pair of rows about 30° apart, and 

 on each side of them are very irregular secondary rows, and numerous 

 miliary granules, which surround the areolae and cover the whole 

 surface. In a very good specimen, lent us by Dr. Wright, there are 

 thirteen plates in an interambulacral row. In the upper ones the 

 length is equal to the breadth, but its proportion decreases towards 

 the periphery, on which the plates are nearly three times as broad 

 as they are long (fig. 4). Each plate bears a perforated primary 

 tubercle, which is placed on a small smooth boss near the lower 

 margin, and, besides, seven or eight much smaller secondary tubercles, 

 perforated and surrounded by the granules. The tubercles on 

 the ambulacral areae are no larger than the secondaries on the 

 other portions. They are numerous, about tliirty-three or thirty- 

 four in a row, perforate, and all of nearly equal size. One or two 

 are here and there larger than the others, but not in any regular 

 order. 



" The poriferous avenues are narrow, in which the holes are thickly 

 set in triple oblique pairs,^' more oblique above (see fig. 4.) than on 

 the under surface, where the threes are so placed that the pairs 

 appear to be alternately 1, 2, 1, 2, the lowermost of the three coming 

 into line with the uppermost of the next rank, and the middle one 



* In the typical group, represented by Ech. elegans, the pores are not arranged in 

 triple rows ; and if to this be added the small size of the genital plates, (which, with the 

 oculars, form a sort of ring round a large anal aperture,) and the very slight indentations 

 of the edge of the mouth, there would seem to be good reason for separating from them 

 Pedina, which has triple ranks of pores, and for making the latter a sub-genus at least 



