HEXACTINELLA MONTICULARIS. 145 



The lamellae separating these tubes consist of a network of beams, mostly 

 40-100 ti thick, with meshes 100-200 m wide. Some parts of this network are 

 quite irregular, others more regular, with more or less quadratic meshes. The 

 beams generally bear small, broad, sharp-pointed, conic spines (Plate 28, fig. 22). 

 Large, freely terminating, conic protulserances, which are hexactine rays and may 

 be designated as thorns, arise from the beams in many places. In the inner part 

 of the lamellae these thorns are not numerous; they are small, usually 90-200 ^ 

 long (Plate 28, fig. 22). In their superficial part they are more numerous, more 

 or less vertical to the surface of the lamella, and larger, 120-360 n long, and about 

 GO M thick at the base. These superficial thorns are covered with spines similar 

 to those on the beams, but on the whole larger and more densely crowded. 



The superficial skeleton-net (Plate 28, figs. 21, 27), remnants of which have 

 been found in several places, extends paratangentially on the surface. It is 

 rather loose and irregular, and consists of pentactines the lateral rays of which 

 have been more or less soldered together. 



The loose hexactines (Plate 28, figs. 17, 18) found in the interior are probably 

 destined to be soldered together to form the internal skeleton-net. The small, 

 probably young forms have straight, or slightly curved, nearly smooth rays, 70- 

 100 M long and 3 m thick at the base. In the larger, probably older ones (Plate 

 28, figs. 17, 18) the rays are 100-260 tx and more long, nearly cyhndrical, and 

 10-14 n thick. They are, in the same spicule, often unequal and always covered 

 with spines. Most of these spines are small, whilst some, which lie irregularly 

 scattered between the small ones, attain a very large size and measure 10-50 n 

 in length. These large spines, of which each ray bears from five to ten or more, 

 increase in size towards the distal end of the ray. The largest of them bear 

 small secondary spines. Several, usually three, are situated terminally. These 

 are always the largest. The large spines along the length of the rays arise nearly 

 vertically, the terminal ones usually point obliquely outward. 



The pentactines (Plate 28, figs. 19, 21, 27) are situated superficially. Their 

 lateral rays, which form the superficial net, are 120-200 n long, 4-10 n thick at the 

 base, and slightly attenuated towards the end. They are covered throughout 

 with vertically arising spines. Young, still free, superficial pentactines (Plate 28, 

 fig. 19) have slender rays and very small spines. Older ones, already incor- 

 porated in the superficial net (Plate 28, figs. 21, 27), have stouter rays and longer 

 spines. 



Of uncinates two kinds, a smaller and a larger, can be distinguished. 



The smaller uncinates (Plate 28, fig. 10), which are very numerous and 



