136 EURETE SPINOSUM. 



curvature being about 20 mm., and may originally have formed part of a cylindri- 

 cal tube about 40 mm. in diameter. 



The colour in spirit is dirty brown. 



The skeleton consists of a continuous net, which pervades the whole lamella, 

 and of loose pentactines, hemioxyhexasters, and scopules. Long, slender 

 rhabds have also been observed, but it is doubtful whether they belong to the 

 sponge. The pentactines form a continuous layer on the intact parts of the 

 surface. Their lateral rays extend paratangentially, their apical ray points 

 inward. The hemioxyhexasters are exceedingly numerous and appear, so far 

 as can be judged by the fragmentary specimen, to occur in dense masses in all 

 parts of the choanosome. The large (perhaps foreign) rhabds lie more or less 

 parallel to the surface. 



The skeleton-net (Plate 29, figs. 18, 19, 23-25) is by no means uniform in 

 structure throughout the thickness of the lamella. In the dermal zone (Plate 29, 

 figs. 18, 23) it is rather irregular, composed of beams 8-40 fi thick, usually 20- 

 35 n, and here its meshes are triangular or irregularly square, not rectangular, 

 and 0.2-0.4 mm. wide. In the gastral zone (Plate 29, figs. 19, 25) on the other 

 hand the network is very regular, composed of longitudinal and transverse 

 beams. The former are 18-50 ^ thick, and on an average about 0.23 mm. apart; 

 the latter are 8-36 m thick, and individually usually extend obliquely but col- 

 lectively from zones which are 0.8-1 mm. apart, and extend transversely, vertical 

 to the longitudinal beams, quite across the whole specimen. With the exception 

 of a few, usually thin ones, which are quite smooth, the beams of the skeleton-net 

 are covered with conic spines, 2.5-8 m high, mostly 5-6 m- The spines of the thin 

 and thick beams are nearly equal in height l)ut differ, often very considerably, 

 in breadth, those on the thicker beams being usually much stouter than those on 

 the thinner beams. Freely terminating rays of the hexactines, by whose concres- 

 cence the network seems chiefly to be formed, arise from the beams in many 

 places. These spine-Uke protuberances are thinner than the beams of the net- 

 work and are only 4 n thick. Here and there local thickenings are observed in 

 the beams. Cylindroconic, terminally rounded spines attaining 25 n in length 

 and 9 Ai in thickness arise from these thickenings. These spines are parallel to 

 the surface of the sponge and the thickenings from which they arise also chiefly 

 extend in this direction. The thickenings with their spines have a cockscomb- 

 like appearance (Plate 29, fig. 24). The comparison of a number of these struc- 

 tures has convinced me that they are in truth hemioxyhexasters which have been 

 soldered to the growing skeleton-net and the rays of which have been secondarily 



