SoLLAS — On VetuHna stalactites [O.8.), ^"c. 491 



except in good preparations, the true nature of the node. This sort 

 of union leads to a regular disposition of the corpuscles in successive 

 series, the spined heads, layer after layer, pointing in one direction, 

 and the rayed ends in the other. 



As a piece of engineering this construction is perfect : nothing 

 could be better adapted to resist pressure in a given direction; the 

 successive corpuscles are superposed like so many arches, with the 

 solid centrum as a key-stone, and as a place of abutment for the 

 bases of the arches -which succeed. The strength of the network is to 

 be measured by the strength of the rays to resist pressure in the 

 direction of their length ; no better disposition of them could be made 

 than this, and the result is a skeleton far more resistant than that of 

 any other Lithistid. I^o other Lithistid that I am acquainted with 

 can withstand strong pressure between the fingers, but no pressure 

 that I can bring to bear in this way causes the least sign of yielding 

 in the skeleton of Vetulina. 



Although the mode of union described is the prevailing one, it is 

 not exclusive. Sometimes a ray terminates by abutting on the base of 

 a spine, and sometimes by clasping the side of a neighbouring ray. The 

 flattened corpuscles (type 3) seem never to serve as abutments for 

 the rays of other corpuscles, but are always united to others, clasping 

 them at any point indifferently, though the rays more usually offer 

 them points d^appui than the centra. 



Since no spines which seem to mark points of union are present in 

 the second kind of corpuscles, and additional rays are, we may con- 

 jecture that these unite with the typical corpuscles in the typical 

 manner, but that they do not receive any rays in return themselves. 

 Both the second and third kind of corpuscles may be looked upon as 

 supplemental, putting in an appearance where a gap requires to be 

 filled up. The third kind may be not only supplemental, but sub- 

 sequent growths arising after the general completion of the network, 

 and thus comparable to the secondary spicules, which appear as an 

 aftergrowth in the network of some Dictyonine Hexactinellids. 



The essential characters of the skeleton of Yetulina are obviously 

 present in the fossil Anomocladina. They are to be recognised in 

 Zittel's representations of the Anomocladina structure, as presented in 

 sections, and by isolated corpuscles ; and in sections of actual speci- 

 mens of fossil sponges of this group they are still more unmistakable, 

 if possible. I owe to the kindness of Colonel H. C. Grant a valuable 

 collection of Astylospongidse, and in the slices fz'om these, which I 

 have had prepared by Mr. Cuttell, all the essential features of the 

 Yetulina skeleton are visible. Of course there are differences : the 

 coi-puscles of those Silurian species which I have examined are rela- 

 tively larger ; the spined heads are replaced by thickened stumps, 

 from which only a few spinelets proceed, and the place of abutment 

 is rather upon these stumps, which appear like an elongation of the 

 centrum, than immediately upon the middle of the centra. The mode 

 of arrangement is, however, the same ; the spined heads point to the 



K.l.A. PROC, SER. II., VOL. IV. — SCIEXCK. 2 X 



