REPORT ON THE HEXACTINELLIDA. 35 



bent prongs (PI. LXXIX. figs. 4, 5 ; PI. LXXXIV. fig. 4). These finely pointed prongs 

 often occur over the whole branch (PL XCII. fig. 5), or on special portions of the same 

 (PI. LXXXLV. figs. 3, 5). I am, on the whole, inclined to regard the scopulse not as 

 pentacts, but rather as diacts or monacts. 



Tetracts. 



If two of the six rays of a hexact on the same axis are not fully developed, simple 

 cruciform spicules arise, with four rays in the same plane. Such regular tetracts occur in 

 the outer, and in the inner (gastral) membrane of many Rossellidse ; as also on the lower 

 end of the body of Hyalonema, in the quadrate latticework of the main supporting 

 framework in many Euplectellidse, and in many other species. Rudiments of the two rays 

 which have not been fully developed can be detected at the nodes of intersection. Such 

 regular tetracts are either quite smooth, or uniformly rough (PL LVI. fig. 5), or beset 

 wdth knobs. Many, however, are only rough or knobbed terminally. Some are more or 

 less uniformly pointed, while others are truncated, rounded, or provided with a knob-like 

 thickening. The four rays are not always equally long (PL III. fig. 28 ; PL XXXV. fig. 8). 

 In many tetracts they are wholly or partially curved, and that either in the plane of 

 the cross (PL III. fig. 27; PL XXXI. fig. 15) or in a spherical form (PL LIV. fig. 7). 

 A few cases occur, which seem to have arisen, not by the abortion of two opposite 

 hexact rays, but of two rays at right angles. In such forms only one of the three typical 

 axes is fully developed, and of the two others only one ray of each has remained 

 (PL III. fig. 20). 



Triacts. 



The three rays of a triact usually lie in the same plane, and consist of two rays 

 belonging to one axis with a third at right angles to these. They are either straight or 

 slightly bent, quite smooth or terminally rough, besides being frequently swollen and 

 knobbed. While the rays of the main axis are generally equal, the unpaired third ray is 

 usually distinctly shorter (PL III. figs. 12, 25), and seldom larger (PL III. fig. 26) than 

 the others. The insertion of this unpaired ray is often opposite a prong, which doubtless 

 represents an abortive fourth ray (PL III. figs. 19, 28). If the unpaired ray is very long, 

 and the two others are bent towards it, a peculiar anchor form results, such as may be 

 seen among the prominent lateral, and basal-tuft spicules of many Hyalonematidse, such 

 as Pheronema, PoUopogon, and Semperella (PL XLVIII. fig. 14a). 



Rarely it happens that all the rays are markedly curved (PL XVI. figs. 3, 4), and 

 correspond in position to three of the edges of a cube. 



