308 



ERYLUS ROTUNDUS. 



The thirteen sponges described above obviously form a systematic, though 

 far from a homogeneous, group, the specimens comprising it differing not incon- 

 siderably from each other in several respects. In three specimens the cortical 

 armour is chiefly composed of microrhabds, the aspidasters in it being but few 

 and confined to its external part. In the other ten the cortical armour is 

 composed chiefly of aspidasters, and the microrhabds which take part in its 

 formation are confined to the external part. In the three specimens the 

 microrhabds are considerably larger and the acanthtylasters and aspidasters 

 smaller than in the ten. Among these ten there is one which has smaller 

 oxyasters and relatively thinner aspidasters than the others, and which pos- 

 sesses aster-like rhabd-clusters, a kind of spicule not observed in any of the 

 others. Thus three secondary groups, megarhabda (armour chiefly composed 

 of microrhabds, microrhabds large, aspidasters small), typica (armour chiefly 

 composed of aspidasters, microrhabds small, aspidasters large, without rhabd- 

 clusters), and cidaris (armour chiefly composed of aspidasters, microrhabds 

 small, aspidasters large, with rhabd-clusters) can be distinguished. 



Two of the specimens of group megarhabda, which come from the south 

 coast of Molokai, are fairly identical, while the third, which comes from the 

 coast of Kauai, has a more grayish colour, smaller rhabd megascleres and micro- 

 rhabds, considerably larger triaenes, larger oxyasters, and thinner aspidasters. 

 Thus two somewhat different forms (A and B) are contained in the group 

 viegarhahda. 



In five of the nine specimens of group typica most of the rhabd megascleres 

 are amphistrongyles or very blunt amphioxes; in the other four most of these 

 spicules are sharp-pointed amphioxes. Four of the five specimens with chiefly 

 amphistrongyle rhabd megascleres, which come from the south coast of Molo- 

 kai, are fairly identical with each other; the fifth, which comes from the north- 

 east coast of Hawaii, is not like these, massive, lobose, but elongate, digitate 

 in shape, having a much lighter colour, thicker rhabd megascleres, less centro- 

 tyle, for the most part simple amphiox microrhabds, somewhat larger euasters 

 and relatively thinner aspidasters. Of the four specimens with chiefly sharp- 

 pointed rhabd megascleres, which all come from the coast of Kauai, three (from 

 Station 4024) are fairly identical, while the fourth (from Station 4128) has a 

 lighter colour, slightly larger microrhabds, much smaller acanthtylasters and 

 longer, considerably broader and thinner aspidasters. Thus four forms (A, B, C, 

 and D) are contained in the group typica. 



The surroundings of all the different forms of the same group must have 



