182 Dr. T. Wright on Fossil Echinoderms 



right angles with the longitudinal axis of the test ; the postero- 

 lateral pair are directed obliquely backwards at an angle of 55°; 

 both pairs lie in rather deep valleys, and the poriferous zones 

 contain from twenty-eight to thirty pairs of pores in each avenue. 

 The peripetal fasciole (1 a) makes an angular zigzag track, closely 

 embracing the apices of the ambulacral areas. In the space which 

 it bounds on the fore part of the shell, having for its base the 

 antero-latcral areas, and its apex the anteal sulcus of the single 

 ambulacrum, a number of large perforated tubercles set on cre- 

 nulated eminences are arranged in groups (1 c), the areolas of these 

 tubercles are surrounded by small granules, and amongst them 

 smaller tubercles are interposed ; a few large tubercles occupy 

 the angles between the apices of the antero-lateral and postero- 

 lateral pairs, and likewise in the angle formed between the pos- 

 tero-laterals themselves ; the rest of the dorsal surface is covered 

 wath small tubercles closely set together ; each tubercle is perfo- 

 rated and raised on a crenulated eminence (1 c), and surrounded by 

 a smooth depressed areola; the base is shghtly convex; the sternal 

 portion of the single interambulacrum is rather prominent be- 

 hind, but slopes gently towards the mouth ; it has an elevated 

 ridge in the centre, and is covered with tubercles of a larger size 

 than those of the upper surface, and which are arranged in re- 

 gular rows. The subanal fasciole is of considerable extent, it 

 forms a semicircle which passes across the most prominent part 

 of the base, and sends its cornua upwards at a considerable 

 distance from the anus ; the basal portions of the pairs of the 

 interambulacral areas are covered with tubercles similar to those 

 on the sternal part ; a naked track corresponding to the postero- 

 lateral areas separates these tuberculated portions of the base. 

 The mouth is situated near the anterior border, it is widest in 

 the transverse diameter, and has a thick projecting under lip ; 

 the terminations of the ambulacral areas surround the mouth, 

 and form poriferous zones around that opening : the anus is 

 situated beneath the margin in an oblique truncation of the 

 posterior border; the opening is much crushed in our specimen, so 

 that its foi-m is not discernible. The apical disc (1 b) is placed near 

 the centre of the back, about y%ths of an inch before that point : 

 the madreporiform tubercle is small and pyriform, and is situated 

 behind the four genital pores : the margin of the shell is thin 

 and acute. 



Affinities and differences. — The breadth of the test and the de- 

 pression of the dorsal surface thereof, with the cm-ve forwards 

 in the ambulacral areas, and the depth of the anteal sulcus, form 

 a group of characters by which Brissus latus is readily distin- 

 guished from its congeners. Out of the seven fossil species, 

 registered but not described in Agassiz and Desor's Catalogue, 



