SKELETAL STRUCTURES. C9 



usually smooth, but in some instances they are moniliform as in Geodites Wrightii, 

 Hinde (Fig. 3, i). These trifid spicules are frequently radiately disposed, so that 

 the head-rays are extended near the outer surface of the Sponge, and hence they 

 have been termed zone-spicules. They are abundant in the existing Geodia, Lam., 

 and allied genera, and in the fossil Geodites, Carter. 



In another form, the spicule is modified in a reverse way to that of the trifid 

 spicules just described, for the ray corresponding to the elongated shaft is reduced 

 to a short blunt process, whilst the head-rays are greatly developed. They are 

 usually furcate, and the rays extend in a generally horizontal direction, forming a 

 right angle with the diminutive shaft (Fig. 3, d). These spicules, equally with 

 those in which the shaft is elongated, are disposed at or near the outer surface of 

 the Sponge. They have been referred to the genus Stellettites, Carter. 



In some of the detached spicules of this group, from Carboniferous strata, only 

 two head-rays have been developed, as in Geodites deformis (PI. V, figs. 4d,g), and 

 similar spicules also occur in the Upper Greensand. 



3. Lithistid Spicules. — There is such great diversity of form in the spicules 

 of lithistid Sponges that no single example can be quoted as typical of the group 

 as a whole. The skeletal-spiculcs forming the body of the Sponge exhibit the 

 greatest amount of variation, whilst in many instances the spicules of the dermal 

 layer are extremely regular in form, and present a striking contrast to the skeletal- 

 spicules even of the same Sponge. In what is regarded as the highest developed 

 group of lithistids, the skeletal-spicules consist of four rays, disposed in the same 

 manner as those of tetractinellid spicules, and with an axial canal in each ray ; 

 whilst in other divisions the skeletal-spicules give off irregularly secondary rays 

 from a main stem, in which there is a single unbranched canal. But it frequently 

 happens that the dermal spicules of Sponges with this peculiar form of skeletal- 

 spicule are distinctly tetractinellid in type, thus indicating a certain amount of 

 relationship between these divisions. Professor v. Zittel has divided lithistid 

 Sponges into the four families of the Rhizomorina, Megamorina, Anomocladina, and 

 Tetracladina, according to the respective characters of their skeletal-spicules. 



(a) Rhizomorina. — In this family of lithistids the skeletal-spicules usually consist 

 of a slender, curved main axis, from which minute, twig-like branches and spinous 

 processes are given off irregularly. The extremities of the branches, where they come 

 in contact with adjoining spicules, are furnished with minute, flattened facets. A 

 simple axial canal can sometimes be seen extending either partially or throughout the 

 length of the main axis (Fig. 4, b), but there are no diverticles from it into the 

 branches. Typical spicules of this character are present in Seliscothon Mantelli, 

 G-oldfuss sp. (Fig. 4, a, b). In another form there is a short, relatively stout, main 

 axis, not infrequently smooth, with diverging branches and spines from both ends, 

 which are likewise facetted at their extremities. This type is well shown in the 



