from the Island of Malta. 87 



to the anus. Four genital pores, the anterior pair smaller and 

 nearer each other than the posterior pair. The madreporiform 

 tubercle situated between the posterior genital openings ; five 

 perforated ocular plates placed before the genital pores and alter- 

 nating with them. This genus contains a greater number of 

 living than of fossil species ; the existing forms are limited to 

 the seas of warm latitudes ; the fossil species are found only in 

 the tertiary rocks. 



Brissus latus (Wright, n. sp.). PI. V. fig. 1 a-c. 



Test convex and much depressed above ; transverse and antero- 

 posterior diameters nearly equal ; ambulacral areas of nearly 

 equal length ; the single ambulacrum lodged in a deep anteal 

 sulcus ; antero-lateral pair curved gently forwards ; peripetal 

 fasciole very zigzag and angular ; apical disc y^ths of an inch 

 before the centre of the disc ; base slightly convex ; sternal 

 process of the single interambulacrum raised before the anus, 

 having a central elevated ridge and numerous large tubercles 

 arranged in regular order on its surface ; subanal fasciole 

 enclosing a space 1 T 7 ^ inch in diameter; anus situated in an 

 oblique truncation below the margin ; mouth y^ths of an inch 

 from the anterior border; the large tubercles of moderate 

 size. 



Dimensions. Antero- posterior diameter 4^ inches, transverse 

 diameter 3-f$ inches ; height at the vertex 1 inch. 



Description. This Brissus is readily recognized by its broad 

 and depressed dorsal surface ; the ambulacral areas form deep 

 depressions in the test ; the single ambulacral area lies in an in- 

 considerable valley on the dorsal surface, but forms rather a deep 

 anteal sulcus ; the antero-lateral pair curve gently outwards and 

 forwards, forming an angle of 21, with the transverse line at 

 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-lateral 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 (Ic), the arrolas of tlu <r 

 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- 



VOL. II. H 



