SKELETAL STRUCTURES. . 69 
usually smooth, but in some instances they are moniliform as in Geodites Wrightii, 
Hinde (Fig. 3, 7). 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 Geodttes deformis (Pl. V, figs. 4d, 9), and 
similar spicules also occur in the Upper Greensand. 
3. Liraistm Spicunes.—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 
asa whole. The skeletal-spicules 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 adjoiming 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, 
Goldfuss 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 
