90 Dr. T. Wright on Fossil Echinoderms 



Brissus oblongus (Forbes MSS., n. sp.). PI. V. fig. 2 a-c. 



Test oblong, depressed before, elevated behind; dorsal surface 

 convex ; anterior border rounded, with a slight anteal sulcus ; 

 antero-Iateral ambulacral areas slightly bent forwards, and 

 nearly forming right angles with the longitudinal axis ; pos- 

 tero-lateral ambulacra make an angle of 68; the anterior are 

 shorter than the posterior pair ; posterior border produced and 

 truncated : anus large, oval, and placed high up : base convex, 

 sternal portion prominent, greatest width across the base of 

 the postero-lateral ambulacra. 



Dimensions. Antero-posterior diameter 2 T \j inch, transverse 

 diameter 1^ inch, height ly 1 ^ inch. 



Description. This small Brissus has an oblong form, and is 

 rounded before and truncated behind ; the anterior half of the 

 test is more depressed than the posterior half ; the single inter- 

 ambulacrum rises into a ridge-like eminence on the back, and the 

 sternal portion thereof is much inflated at the base, so that the 

 greatest height of the test is in this region. The antero-Iateral am- 

 bulacra (2 a) are shorter than the posterior pair, and are extended 

 across the test nearly at right angles to the longitudinal axis ; the 

 postero-lateral ambulacra are longer than the anterior, and form 

 angles of 68 ; the petaloid portions of both areas are depressed ; 

 the anterior pair have eighteen pairs of pores, the posterior pair 

 have twenty-four pairs of pores in their poriferous zones : the 

 single ambulacrum is not lodged in a rudimentary anteal sulcus, 

 and is nearly on a level with the contour of the test, the front 

 and cheeks of which are convex, with four groups of larger tuber- 

 cles in this region ; the sides slope obliquely downwards to the 

 border, which is obtuse : the single interambulacrum is raised 

 into a ridge above, and produced into a blunted caudal process, ob- 

 liquely truncated behind: the base (2 b) is convex, chiefly from the 

 arched form which the sternal portion of the interambulacrum 

 assumes : the mouth is large, near the anterior third of the base : 

 the anus is of an elliptical form, and occupies more than the 

 upper half of the truncated portion of the posterior border. The 

 peripetal fasciole (2 a) closely embraces the ambulacral pairs, and 

 makes three angles in passing over the anterior interambulacra ; 

 the subanal fasciole (2 b) describes a heart-shaped outline, its base 

 is near the anus, and its apex touches the prominent point of the 

 sternum ; the space thereby circumscribed is filled with tubercles 

 having a definite arrangement. The apical disc is small, with 

 four genital pores, the posterior pair being much larger than the 

 anterior pair; the tubercles (2c) on the anterior interambulacra are 

 much the largest, those on the rest of the back are small and 

 very uniform in size, whilst those on the sternum and the sides 



