810 W. J. SOLLAS ON THE STRUCTURE AND 



sure to do, a littlo above or below its equator, will intersect the 

 canals and their surrounding network obliquely. 



The arrangement about the longitudinal canals differs from the 

 proceding, though it appears partly to result from it. Walking up 

 one of these canals, one would pass alternations of knotted ends, or 

 of bands of knotted ends, and series of smooth trabecule, resembling 

 altogether a chain made of links of two alternating patterns ; i. e. 

 we should make our way from one spicular series to another. 



It only remains to repeat that the arrangements just described 

 are subject to very considerable modifications ; but these, however 

 great they may be, leave the general tendency always observable. 



The simple acerate spicules (p. 808) also present us with a more or 

 less obvious arrangement, generally lying in groups parallel with 

 one another and with the direction of the adjacent longitudinal or 

 radiating canals. 



This exhausts, so far as my observation goes, the minute structure 

 observable in the phosphatic specimens from the Gault ; and it will 

 now be worth while to determine how far similar characters are to 

 be detected in species from other localities and in different states of 

 fossilization. 



Specimens from the Haldon Greenland near Exeter. S.jpyriformiSi 

 Sow. (PI. XXV. fig. 1), S. cylindrica (PI. XXV. fig. 4) and emica, Court., 

 S. Fittoni, Mich. (PI. XXV. fig. 6). — These fossils are not much else 

 than the deciduous skeletons of the sponges they represent, un- 

 altered to any great extent by processes of fossilization. Scarcely 

 any foreign material has entered to fill up the canals and interstices 

 of the interior ; and thus it happens that it is next to impossible to 

 prepare transparent sections from them : the brittle siliceous net- 

 work breaks away in the processes of cutting and grinding down, 

 and none but thick, almost opaque, slices can be procured. For- 

 tunately, however, this perfection of preservation has its own 

 advantages ; for, owing to it, we can dispense with section-cutting 

 and preliminary preparation altogether ; with no other apparatus 

 than a low-power microscope, say of 60 diameters, and a common 

 Haldon Siplionia, we can, by examining the latter under the former 

 with reflected light, solve at once the characters and affinities of this 

 long misunderstood genus. That the solution has not come before 

 is due to the ignorance in which we have been left so Ions: regarding 

 the nature and existence of the Lithistina. Almost directly we 

 have attained a knowledge of these we have also arrived at a 

 solution of Sijphonia. 



On examining the natural surface of the Haldon specimens under a 

 power of 60 diameters, one perceives that the spaces between the pore- 

 areas are entirely occupied by a skeletal network (PI. XXVI. fig. 4) 

 possessing the truo Lithistid structure. The quadriradiate spicules 

 interlocked by their tubercular extremities are plainly visible 

 throughout, and may be viewed here as solid objects of three 

 dimensions, and not merely as linear figures drawn in the plane of 

 a section ; as regards outward appearance, many of these spicules are 

 as whole and perfect, and clean and vitreous, as those of a recently 

 dead sponge. 



