Siliceous Sponges. 



117 



of their rays, so as to produce a very strong spicular meshwork 8pong«c. 

 capable of enduring fossilization. The suborder has been divided GALLEEY 

 into the following families : — {a) Tetracladina ; in which the ^ ^1. . 

 spicules have four equal or subequal rays with their ends produced ■v^all-oasea 

 into twig-like processes, which intertwine with those of adjoining 7 & 8, 

 spicules and produce rounded nodes (Fig. \1\c, d). To this ■^*^^" 

 family belong such well-known sponges as Siphonia, Hallirhoay H-15, 

 and Callopegma. {h) Eutaxicladina. The skeletal spicule in this 

 family has a thickened central node, from which three or more 

 rays are given off. The rays are somewhat expanded at the ends, 

 so as to clasp the nodes of adjoining spicules, and form thereby 

 a close meshwork (Fig. 170A). The prineipal genus in this 



Fig. 170. — Different forms of siliceous spicules of fossil lithistid sponges. («, b) 

 Skeletal spicules of a rhizomorine lithistid, Seliscothon. {c) Skeletal spicule 

 of Cnemidiastrum. (d) Skeletal spicule of Aulocopium. {e,f) Spicules of 

 a megamorine lithistid, Boryderma. {g) Spicule of the megamorine lithistid 

 Carterdla. (h) Spicules of the eutaxicladina lithistid, Astylospongia. 



«, X 40; 6, X 40; e, x 40 ; ^, x 40 ; ^, x 26 ; /, x 20; g, x 20; 

 h, X 40. 



family is Astylospongia, Eiimer. (<?) Anoraocladina. Th« skeletal 

 spicule consists of a rod-like axis, provided with an axial canal 

 and with a thickened node at each end, from which a variable 

 number of rays are given off. The termination of the rays and 

 their mode of union are the same as in the preceding family 

 (Fig. 171 J). Cylindrophyma and Melonella are the principal 

 genera, (rf) Megamorina. The spicules in this family are rela- 

 tively large, curved, and irregularly branching rods, with a simple 

 axial canal. These are in some cases intertwined together like 

 wicker-basket work^ in other cases the ends are expanded so as 

 to clasp adjoining spicules (Fig. 170^,/). The surface layer 

 in some of the sponges consists of regular trifid spicules. 



