72 BRITISH FOSSIL SPONGES. 



these nodes are given off simple and furcate rays with expanded ends as in the 

 spicules with single separate nodes. In the normal elementary spicules of this 

 family no definite canals have been observed, and even in the spicules of the sole 

 existing representative, Vetulina stalactites, Os. Schmidt, canals do not appear to 

 be present ; at all events they are not mentioned or figured by Sollas 1 in his recent 

 paper on this species. In the twin spicules of Cylindrophyma, however, as first 

 pointed out by Dr. Linck, 2 there is a well-defined simple axial canal in the short 

 axis connecting the two nodes of the spicule, and this canal can occasionally be 

 traced into the central portion of one or other of the nodes, thus proving that the 

 twin nodes and the connecting axis form a single elementary spicule. The canal, 

 however, does not give out branches into the rays of either of the twin nodes. In 

 many of the spicules of Cyli?idrophyma Steinmanni, 5 Linck, there is a gradual 

 diminution in the size of the nodes and the rays proceeding from them, which 

 become also more spinous, and they then resemble spicules of the Rhizomorina 

 family, .and can scarcely be distinguished from those of Cnemidiastrum Hoheneggeri, 

 for example (Fig. 4, c). On the other hand, the rays given off from the nodes of 

 Anomocladina spicules are very similar to the branches given off from the main 

 axis of Megamorina spicules, and the principal differences in the spicules of these 

 two families consist in the fact that the spicular rays or arms in this latter 

 proceed irregularly from an elongated axis, whilst in the former they proceed 

 from a central node. 



Professor ZitteP has lately defined the elementary spicule of the Anomocladina 

 family as consisting of simple, straight, or curved rods more or less branched at 

 both ends, and forming nodes by the union of the branched ends of proximate 

 spicules. Judging, however, from the characters of the spicules of the existing 

 Vetulina stalactites, 0. Sell., in which the central node, and the rays proceeding 

 from it, evidently form a single structure, as figured by 0. Schmidt 5 and by Sollas, 6 

 and from the further fact that in the fossil examples of Cylindrophyma, the spicules 

 produced by the disintegration of the skeleton likewise consist of nodes or centra 

 with radiating rays, the evidence seems to be strongly in favour of the theory that 

 these bodies are really the elementary spicules, and not, as suggested by Professor 

 Zittel, compound bodies formed by the union of several simple rod-like spicules. 



(d) Tetrad adina. In this family the skeletal-spicules are four-rayed; the 

 rays or arms diverging from a common, non-inflated centre, at an angle of 120° 

 from each other; they are either smooth (Fig. 5, c), or covered with small tuber- 

 cles (Fig. 5, e). Near their extremities the rays divide into two or more branches, 



1 ' Proc. Eoyal Irish Academy,' 2 ser., vol. iv, No. 4, p. 486. 



2 ' Neues Jahrb., &c.,' 18S3, ii Band, lster Heft, p. 59. 



3 Ibid. 4 'Neues Jahrb., Ac.,' 1884, Band ii, p. 75. 

 5 ' Die Spongien des Meerbusen von Mexico,' p. 19, pi. ii, fig. 9. 



c ' Proc. Eoyal Irish Academy,' 2 ser., vol. iv, p. 486, pi. iv. 



