BRITISH FOSSILS. 3 



Becoiidary tubercles, and a few granules also are interspersed. On 

 the plates of the upper suiface these granules surround the primaries, 

 but on the lower ones are only occasionally to be met with. 



The primary tubercles in the interambulacral spaces are larger 

 than those on the ambulacra, but not quite so many in a row (about 

 sixteen or seventeen), and from about the upper third of the seg- 

 ment there are secondary tubercles, likewise perforated, but very 

 much smaller. There is a single row of these outside the primaries, 

 and a double row down the middle. These secondary tubercles are 

 largest at the turn of the margin, and decrease rapidly in size 

 above or below that point. Each plate, at least on the more pro- 

 minent parts of the test, bears two or three secondary tubercles on 

 each side of the primary one. Of these the most conspicuous are 

 placed on a line with the primary tubercle, the others occupy the 

 angles of the plates, and are interspersed with numerous granules, 

 which only on the upper plates surround the primary bosses. These 

 bosses are of a transverse broad oval form, and for the greater part 

 of each row are confluent at the base. They are flat round their 

 base, then rise steeply to the crenulated edge, which is divided by 

 about fifteen notches, and support small strongly perforated tuber- 

 cles. They increase very regularly in size outwards, and radiate 

 regularly from the apex, instead of divaricating widely, as in the 

 recent D. turcarum, and then converge rapidly on the under 

 surface, so as to leave a depressed space between the ambulacral 

 tubercles occupied by the pores. There are about seventy pairs of 

 pores in each row, which are disposed in a single series, and run in 

 nearly a straight line from the apex to the circumference ; below 

 this they undulate a little, and close to the oral margin about 

 twenty pairs fall into ranks of threes. 



Each pair of pores forms an ovoid, and is incKided in a ring, and 

 an elevated tubercle or ridge separates the pores^ forming as it were 

 a handle across the elliptical space. 



The apical disk is small compared with that of other species, in 

 our largest specimens not half an inch across. It is composed of 

 five obovate genital plates, and five minute ocular plates ; the 

 genital plates extend outwards much beyond the oculars ; they are 

 covered by large granules, and are perforated nearer the outer than 

 the inner margin. 



The right (anterior?) plate, which bears the madreporiform 

 tubercle, is decidedly larger than the rest. The anus is roundish, 

 not quite regular in shape. The mouth is larger than in our figure, 

 measuring, from point to point of the ten deep notches, at least 



