Siliceous Sponges. 



115 



as rusty bands traversing the chalk or flint. Frequently, also, Spongw. . 

 the siliceous skeleton has been altogether dissolved away, leaving GALLBEY 

 merely an empty cast in the matrix, or very commonly the" cast -^^i gjvi- 

 has been refilled with calcite, as in many of the sponges from the Wall-cagai 

 Jurassic strata of Wiirtemberg. I* .•» 



The Silicispongiae are arranged under the following suborders, 

 according to the form and characters of their skeletal spicules. 



I. Monactinellidae. — The spicules in this group are principally 

 simple, straight, or curved rods or needles, pointed at both ends ; 

 they may be also styliform, pin-shaped, or cylindrical, and their 

 surfaces are smooth, spinous, or moniliform. With these are 

 associated smaller flesh-spicules, having the form of hooks, clasps, 

 anchorets, etc. (Fig. 168). The spicules in these sponges are 



Table. 



cases 



11-15. 



Fig. 168.— Different forms of siliceous spicules of fossil monactmelhd sponges. 

 [a, b, e) Fusiform acerate spicules. (c) Vermiculate acerate spicule of 

 ScoUorhaphis. (0) Vermiculate, smooth acerate. (/) Acerate spicule witH 

 open canal, (a) Simple curved cylinder with axial canal completely enclosed. 

 (A) Cylinder with microspines. (i) Moniliform cylinder, (j) ^'^'''^f^'S- 

 (k\ n) Acuate or styliform. [I, m) Spinulate or pm-shaped (« TibieUa 

 spicule, {p) Moniliform acuate. (?) Bihamate spicule, (r) Clasp-boot 

 flesh -spicule, (s, t) Equianchorate flesh -spicules. 



a, X 26; b, x 100; c, x 40; d, x 66; e, x 26; /, >< ^Oj Vn?^ 

 h, X 114; i, X 40; /, x40; k,xiO; ?, xl3; ni xUi; n, x 114, 

 0, X 114; p, X U; ?, x 40; r, x 114; s, t, x 114. 



either inclosed by homy fibres or loosely arranged in the soft 

 tissues. Though the characteristic spicules of this group are met 

 with abundantly in a detached condition in siliceous deposits of 

 Tertiary age for the most part, the sponges themselves are rarely 

 preserved as fossils. . , 



II. Tetractinellid^.— The characteristic skeletal spicules in this 

 group have four rays extending from a common centre. In the 



