SKELETAL STRUCTURES. 71 
present in the same spicule (Fig. 4, ¢). The spicules of this family are relatively 
large, and their surfaces smooth. As a general rule, only a simple canal is present, 
and this only partially, in the main axis of the spicule, but in the great majority of 
the spicules the canal is not distinguishable in the fossil condition. In the genera 
Isorhaphinia, Zitt., Carterella, Zitt., and Nematinion, Hinde, the spicules are 
threadlike and variously notched. In Carterella the simple spicule has slender, 
curved, and twisted extremities (Fig. 4, 9), whilst in Isorhaphinia the spicule has 
slightly tumid terminations. The greatest diversity of form is shown in the 
spicules of Megalithista, Zitt., Doryderma, Zitt., and Heterostinia, Zitt., which are 
curved so variously, and give off lateral and terminal branches in such an irregular 
manner that it is difficult to find any two spicules closely alike (Fig. 4, e, /). 
(c) Anomocladina.—The elementary skeleton-spicule of this family of lithistids 
consists of a thickened, rounded, or compressed central node, from which a variable 
number of rays (from three to nine) radiate in different directions. The rays are 
usually smooth, simple, or occasionally furcate, and they terminate in expanded 
surfaces with even or digitate margins (Fig. 5,a). In the genus Cylindrophyma, 
Fic. 5.—Various forms of fossil skeletal- and dermal-spicules of lithistid Sponges. (a) Elementary 
spicule of Anomocladina lithistid, Mastosia Neocomiensis, Hinde, from the Upper Greensand of 
Warminster, Wilts. (2) Paired or twin spicule of Anomocladina lithistid, Cylindrophyma mille- 
porata, Goldf., sp. (¢) Skeletal-spicule of Tetracladina lithistid, Callopegma acaule, Zitt. (a) 
Skeletal-spicules of Callopegma Schlenbachii, Zitt., showing their mode of union with each other. 
(e) Skeletal-spicule of tetracladine lithistid, Plinthosella squamosa, Zitt. (f) Dermal spicule of 
lithistid, showing the canals. (g) Another dermal spicule, showing the rudimentary shaft. (%) 
Dermal spicule of lithistid, in which no shaft is developed, but traces of the canals remain. 
(2) Dermal spicule of lithistid with fureate head-rays. 
Zitt., many of the elementary spicules are of a twin-like character, consisting of 
two distinct nodes united by a short cylindrical axis (Fig. 5,6). From each of 
