SKELETAL STRUCTURES. 



71 



present in the same spicule (Fig. 4, e). 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., Carter ella, Zitt., and Nematinion, Hinde, the spicules are 

 threadlike and variously notched. In Carterella the simple spicule has slender, 

 curved, and twisted extremities (Fig. 4, g), whilst in Isorhaphinia the spicule has 

 slightly tumid terminations. The greatest diversity of form is shown in the 

 spicules of Megalithista, Zitt., Dory derma, 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,f). 



(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, 



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Fig. 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. (£) Paired or twin spicule of Anomocladina lithistid, Cylindrophyma mille- 

 porata, Goldf., sp. (c) Skeletal-spicule of Tetracladina lithistid, Callopegma acaule, Zitt. {d) 

 Skeletal-spicules of Callopegma Schlaenbachii, Zitt., showing their mode of union with each other. 

 (e) Skeletal-spicule of tetracladine lithistid, Plinthosella squamosa, Zitt. (/) Dermal spicule of 

 lithistid, showing the canals, (g) Another dermal spicule, showing the rudimentary shaft, (h) 

 Dermal spicule of lithistid, in which no shaft is developed, but traces of the canals remain. 

 (i) Dermal spicule of lithistid with furcate 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, b). From each of 



