356 BRISSUS. 



fascicle is elliptical, undulating; the anus is placed near the posterior ex- 

 tremity of the peripetalous fasciole. In still younger specimens the peripe- 

 talous fasciole is rectangular, with rounded corners and concave sides, 

 extremely broad (PL XIX. f. ?'), wmle, as it widens with the increased 

 length of the ambulacra, it becomes much narrower. 



The odd ambulacrum carries lour or five large tentacles with lobed dish 

 (PL XIX. f. „') ; the pores of the odd ambulacrum are single, not in pairs ; 

 the other ambulacra are short, straight, well defined, consisting of three and 

 four pairs of pores not yet conjugated (PL XIX. f. 7). In older specimens 

 the posterior edge of the mouth becomes labiate, the anus approaches the 

 subanal fasciole, which sends out a rudimentary anal branch (PL XIX. f. 4), 

 eventually uniting with the peripetalous fasciole, the outline of which be- 

 comes more pentagonal (PL XIX. f. 8), undulating, and elongated with the 

 increasing size of the petaloid ambulacra. The posterior edge becomes more 

 bevelled with age (PL XIX. f. 6), the subanal plastron more prominent, the 

 lateral pairs of ambulacra gradually tend to unite, passing from a strictly 

 Brissopsis outline (PL XIX. f. <s') to one considered hitherto characteristic of 

 Toxobrissus (PL XIX. f. 9). The spines in all young Spatangoids are 

 strikingly larger in proportion to their size than in the adult. 



From bb to 156 tkthoms. 



BRISSUS. 



Brissus Klkin. 1 734. Nat. Disp. 



Echini with elongate test ; apex eccentric anteriorly, often attaining very 

 large size; anterior ambulacrum nearly obliterated, flush with test, or placed 

 in a furrow of little depth. Other petals more sunken ; anterior ambulacra 

 more transverse than the posterior. Anal system moderately large ; tuber- 

 cles, except on each side of anterior furrow, and within the peripetalous fas- 

 ciole, remarkably uniform in size ; peripetalous fasciole exceedingly angular, 

 with a very prominent subanal fasciole ; four genital pores, anterior small, 

 close together, posterior ones larger and distant ; spines very uniform in size, 

 except on the actinal surface and on each side of the bare band, where 

 they are quite long and well developed. 



