BRITISH FOSSILS. 



3 



terior ovarians, and three around each of the oculars. The pits at the 

 junction of three plates are triangular, those at the junction of two, 

 circular. They are never prolonged as linear notches into the substance 

 of the plates. The surfaces of the latter, in a few rare specimens, 

 exhibit indietinct indications of radiated ribbing. The anus itself is 

 subcircular, slightly inclining to transversely oval. It is bordered 

 by a very prominent rim, formed of the elevated margins of the pos- 

 terior ovarian and supra-anal plates. The sutures of parts of the rims 

 appear in many specimens as if consolidated. 



The ambulacra are composed of two slightly sinuous poriferous grooves, 

 and separated by the ambulacral spaces. The latter are narrow, and 

 through a great part, of nearly equal dimensions. Each ambulacral 

 plate bears a marginal tubercle, and one or two smaller ones on its 

 inner or central side. The marginal tubercles, being nearly of an 

 equal size, form a closely ranged border to the ambulacral spaces ; the 

 interspace between the borders is occupied by alternating tubercles 

 of smaller dimensions. The poriferous avenues are depressed and very 

 narrow. The pairs of pores are ranged in single file, except immediately 

 near the mouth, where they are double and triple ranked. Each pore 

 is separated from its fellow by a rather prominent ridge. The inter- 

 ambulacral spaces are, in their centres, three times the breadth of the 

 ambulacrals. They are composed of few plates, some of which bear 

 areolated spaces, surrounding primary spiniferous tubercles. These are 

 borne on elevated prominences, which form a dentated margin around 

 their bases. The primary tubercles are not equal in diameter to the 

 breadth of the rings surrounding them. The edges of these circles 

 are bordered by a rather scattered series of secondary tubercles, usually 

 about seven in number. Of the primary tubercles and their areolae, 

 there are three most conspicuous on each interambulacral area, two to 

 the left, and one, nearly centrical, on the right side. Down the centre 

 of each interambulacral area runs undulating an elevated, but plane 

 space, occupied by tertiary tubercles, closely and rather irregularly set, 

 bordered by the secondary tubercles which bound the rims of the 

 areolae. There are no tertiary tubercles in the spaces between one areola 

 and another of the same vertical series. 



The mouth is round, and not very large, as compared with the dia- 

 meter of the body. Its margin is deeply notched opposite each poriferous 

 suture, and sinuated in the interambulacral intervals. It is bordered 

 all round by a more or less thickened rim. 



In a well-grown specimen there are about 38 pairs of pores in each 

 series, and about 18 bordering secondary tubercles on each side of an 

 ambulacral space. 



As yet the spines are unknown. 



