56 PAL.EOBKISSUS HILGARDI. 
* Palaeobrissus Hilgardi A. Aq. 
Station No. 300. 82 fathoms, Barbados. (^ 
Station No. 295. 185 fathom.'i, Barbados. 
•^(^< 
Fl. XXIV. Figs. G-15. 
At the time of writing the Prehmiuary Eeport on the Echini of the 
Blake cruise in the Caribbean, the specimens of this sjDecies were not 
distinguished in the first examination froiii Palceotropus JosepMnce, to which 
they have, when covered with spines, a general resemblance Itoth in shape 
and coloring (PI. XXIV. Figs. G-8). Palasobrissus is one of the most inter- 
esting generic types collected hy the Blake. From the structure of its 
ambulacra it is closely allied to both Platybrissus, Nacopatagus, and Argo- 
patagus. The ambulacra are not petaloid, as in the former genus. The 
pores of the lateral ambulacra are arranged in straight, double, diverging 
i-ows (PI. -XXIV. Fig. 10). The pairs of minute pores of the abactinal region 
increase rapidly in size towards the extremity of the rudimentary petals, 
the last four pairs being large and well separated, the outer pore of each 
pair slightly larger than the inner one ; the lateral ambulacral petals extend 
nearly to the ambitus (PI. XXIV. Figs. 11, 12). The odd anterior ambula- 
crum is not as prominently developed as the lateral ambulacra, the pores 
remaining all small (PI. XXIV. Fig. 10). There are four genital pores (PI. 
XXIV. Fig. 15), the anterior pair the smallest, separated from each other 
by the well-marked madreporic pores, which conceal the sutures between 
the anterior genital plates. The posterior genital plates are comparatively 
large, adjacent, becoming intercalated with the median posterior interambu- 
lacral plates. The anterior part of these genital plates is perforated by the 
large elliptical posterior genital pores. The structure of the apical system 
of this genus shows how the abactinal interambulacral plates are derived 
from the terminal plates of a compact apical system ; these are not neces- 
sarily the genital plates, but may be intercalated plates formed from the 
subdivision of any of the abactinal interambulacral plates at the apical junc- 
tion of the posterior lateral interamliulacra with the odd interambulacrum. 
The upper part of the test (PI. XXIV. Fig. 10) is covered by small pri- 
mary tubercles of uniform size, somewhat distant, quite regularly arranged 
on the plates, increasing in number towards the ambitus. The intertuber- 
cular space is filled by distant miliaries, also quite uniformly scattered over 
