2 



BRITISH FOSSILS. 



segment, the margin of which is slightly truncate. The dorsal 

 surfaces of the anterior and lateral interambulacral, and of all the 

 ambulacral segments, are convex with a gentle curve, but that of the 

 hinder interambulacral segment is excavated by a deep longitudinal 

 furrow opening into the nearly round apical pit, narrow and steep 

 sided in its upper half, depressed and expanding in its lower 

 portion, where the sides of the segment are also flattened, and more 

 or less declining. The margin of the apical pit is notched with four 

 strongly marked interambulacral notches and five less strongly 

 marked ambulacral ones, the former correspond to the insertions of 

 the four perforated genital plates, and the latter to those of the five 

 ocular ones ; but no specimen has yet occurred exhibiting the minute 

 details of the disk. The whole of the plates of the dorsal surface 

 bear very numerous minute, regular, and nearly equal primary 

 tubercles, perforated, borne on crenulated bosses and lodged in a 

 depression forming an areola about each ; they are separated from 

 each other by granules. These tubercles become still more numerous 

 and slightly larger on the marginal portions of the test, where they 

 are separated from each other by so narrow a line of granules that 

 each appears to be placed within an hexagonal frame. Towards the 

 mouth they gradually increase in size and become more separated, 

 with considerable numbers of granules keeping them apart. The 

 pairs of pores are ranged in the avenues dorsally in single file, but 

 vfhen they turn the margin and pass on to the under surface they first 

 gradually fall into oblique sets of three pairs, and eventually, close 

 to the mouth, into very oblique ranks of threes. The mouth is 

 excentric in the opposite direction from the apical disk, for whereas 

 the latter is excentric towards the posteal extremity, the mouth is 

 excentric tov/ards the anteal margin. The mouth is longer than it 

 is wide, and has rounded shghtly tumid lips ; the spines are minute 

 and very short, subulate and finely striated. 



Dr. Wright has met with a specimen three inches and -p^ths 

 in diameter. A fine example in the Museum of Practical 

 Geology measures 2 inches and -j-\-ths in length by 2 inches -^-^ths 

 in breadth. The greatest thickness is -^^ths of an inch. From 

 vertex to posterior margin it measures 1 inch and -f^a^^s, and to the 

 anterior margin 1 inch and -j-\-ths ; the anterior and antero-lateral 

 ambulacra are -j-\ths of an inch in breadth ; the postero-laterals -j%ths 

 of an inch in breadth. The length of the mouth is -fV^l^s, and its 

 breadth -y^ths of an inch. The breadth of the anal furrow just 

 below the apical concavity is -j-\ths of an inch. 



