BRISSUS. 15 



Genus — Brissus, Klein. 



Body oval or oblong, tumid; dorsal ambulacra sub-petaloid, circumscribed by a 

 peripetal fasciole ; tubercles of dorsal surface all similar ; anus terminal, supra-marginal ; 

 caudal extremity with a sub-anal fasciole. 



The living species of Brissus are chiefly tropical. The fossil representatives of the 

 genus are entirely tertiary. 



1. Brissus Scill^e, Agassiz. Plate II, fig. 4. 



Scilla, De Corp. Mar., pi. iv, figs. 2 and 3. 

 Brissus Scully, Agassiz and Desor, Ann. Sc. Nat., 3d ser., torn, viii, p. 13. 

 Spatangus (Brissus) placenta, Philippi in Erichson's Archiv. for 1845, pt. 1, 



p. 349 ? 



This sea-urchin, one of the largest and most remarkable of all those found fossil in the 

 Crag, varies much in shape, some specimens being oblong, some wide and ovate ; the 

 former are usually high, and strongly subcarinated on the back ; the latter more depressed, 

 but all have the apex strikingly excentric, and the anterior extremity abruptly truncated. 

 The greatest width of the body is nearly on a line with the terminations of the postero- 

 lateral ambulacra. The tubercles of the back are numerous and closely set, and increase 

 gradually in size in the anteal region and towards the apex. The lateral ambulacra are 

 narrow, somewhat linear in shape, and deeply impressed, showing on the surface as four 

 deep radiating furrows, two of which, the antero-lateral ones, stand at right angles to the 

 longitudinal diameter of the shell, whilst the other two, the postero-laterals, are directed 

 obliquely backwards, and form an acute angle at their apical terminations. The latter are 

 a little longer than the former, and contain rather more pairs of pores, the respective 

 numbers in each row being from 27 and 30 to about 30 and 35. The centro- 

 ambulacral space is smooth, or nearly so, in the lateral ambulacra ; but in the odd, or 

 anteal ambulacrum, which, instead of being impressed and sub-petaloid, is linear and 

 plane, or even slightly elevated, it is regularly and minutely granulated, the large granules 

 or small tubercles forming boundary rows. In the lateral ambulacra, the ridges separating 

 the pairs of pores are minutely granulated. The genital disk, usually obscured in fossil 

 specimens, has four genital holes, the two posterior ones largest, and five eye-perforations, 

 remarkable for their peculiar structure. The peripetal fasciole is very distinctly marked. 

 In front of the antero-lateral ambulacra, it includes a wide somewhat semicircular space, its 

 foremost and central portion crossing the shell at a little below half its height. From this 

 point, tracing its course along each side, it runs with a slight angularity to about two 

 thirds of the distance between the anteal and the end of the antero-lateral ambulacrum, 

 before meeting which it makes a single strongly-marked incurved flexure, in this respect 



