BRITISH FOSSILS. 



3 



of the pores there is a very small tubercle or two on each plate, 

 the rest of the surface being covered by minute and closely set 

 granulations ranged in transverse rows. The antero-lateral am- 

 bulacra are undepressed and superficial, and exhibit a slight and 

 graceful curve ; they are composed of an inner series of minute and 

 approximated pairs of pores and of an outer or hinder series, in which 

 the pores of each pair are widely separated and unequal (the outer 

 ones being longest), and connected by a shallow groove. There are 

 about 30 pairs of pores in the petaloid portion of each series ; the 

 petals gradually become narrow and cease about half way doT^m 

 the sides. In the postero-lateral ambulacra, the petals are also 

 plane and undepressed, but are more regularly lanceolate in shape, 

 in consequence of the two series of pairs of pores in each being of 

 nearly similar structure and but slightly unequal in width. There 

 are about 18 pairs in each petaloid avenue. A few tubercles are seen 

 upon the lateral ambulacra. On the interambulacral spaces they 

 are much more numerous, though set well apart and much larger. 

 They are largest on the anterior segments, where they have wide 

 areolae. Their interstices over the whole of the test are occupied 

 by minute granules, which are, however, not so small or so regu- 

 larly arranged as those on the odd ambulacral plates. On the 

 under surface the tubercles are confined for the most part to the 

 interambulacral spaces, and are especially numerous, regular, and 

 closely set on the ovato-lanceolate post-oral space formed by the 

 inferior portion of the hinder interambulacrum. This space exhibits 

 a prominent caudal gibbosity. The tubercles are minute and per- 

 forated ; they are elevated upon crenulated bosses. The spines are 

 unknown. 



The apical disk is small, and is composed of four perforated 

 genital plates, the perforations of which are approximated. In the 

 midst of them is seen the madreporiform body. A fifth genital 

 plate is imperforate. The five ocular plates are all perforated, but 

 very minute. 



The larger of the two specimens figured measures 1 ~\ inch in 

 length, the same in breadth, and one inch in height. The smaller 

 is 1 -V inch in length, the same in breadth, and -p^ths of an inch in 

 height. 



Locality and Geological Position, The Greensand of Blackdown 

 in Devonshire, a formation whose geological horizon is probably 

 about the junction of the Gault and Upper Greensand, if it be 

 not a local condition of the Gault itself The specimens described 

 are in the Museu^ia of Practical Geology. 



