56 PAL.EOBKISSUS 1IILGAEDI. 



* Palaeobrissus Hilgardi A. Aq. 



Station Xo. 300. 82 fathoms, Barbados. 



Station Xo. 295. 165 fathoms, Barbados. 



PI. XXIV. Figs. 6-15. 



At the time of writing the Preliminary Report on the Echini of the 

 Blake cruise in the Caribbean, the specimens of this species were not 

 distinguished in the first examination from Palceotropus Josephines, to which 

 they have, when covered with spines, a general resemblance both in shape 

 and coloring (PI. XXIV. Figs. 6-8). Palaeobrissus is one of the most inter- 

 esting generic types collected by 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 

 rows (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 

 front 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 interambulaera with the odd intcrambulacrum. 



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- 

 <•iil.ii- space is filled by distant miliaries, also quite uniformly scattered over 



