W. J. SOLLAS ON PHAKETKOSPONGIA STKAHANI. 251 



of the same interior diameter as themselves ; while, finally, in rare 

 cases it happens that these spherules join together into a cylinder, 

 and so replace the spicule for a great part of its length. 



Precisely similar spherules, but more variable in size, occur 

 also scattered through the matrix of the fossil, of which one is 

 represented in PI. XI. fig. 14 ; and associated with them are a 

 number of round, almost spherical bodies (PL XI. fig. 15), about 

 0*005 of an inch in diameter on the average, and of the same clear 

 transparent brown colour, with which colour also the chalk-marl 

 immediately surrounding them is faintly stained. On careful ex- 

 amination these are seen in many instances to consist of aggre- 

 gations of smaller spherules (PI. XI. fig. 16). What these bodies 

 and the spherules may be appears very doubtful ; but they certainly 

 have very much the appearance of standing to each other in the 

 relation of spores and sporangia *. In specimens where the matrix 

 is coloured with phosphatic matter, this substance sometimes pene- 

 trates the fibre of the sponge, and, occupying the space between 

 the spicules once filled with animal matter, stains the exterior of 

 the spicules of its own brown colour. 



Thus, to sum up, we find that the spicules are generally replaced 

 by caibonate of lime, and sometimes infested by minute spherules 

 of an unknown nature; that the animal matter of the fibre is also 

 replaced by carbonate of lime, but in a granular condition, and by 

 phosphatic material ; and, finally, that the intermeshes are filled 

 up by the material of the stratum in which the fossil is imbedded. 

 Prom this last fact we may infer that our Cambridge P. Strahani 

 is generally contemporaneous with the bed in which it is found, 

 and not derived from the subjacent Gault. 



Observations on the Classification of fossil Sponges. — The proposal 

 of D'Orbigny to separate all the fossil sponges, except Cliona, from 

 all the recent ones, as an extinct order (" Amorphozoaires a sque- 

 lette testace ") characterized by a hard and resisting calcareous 

 network, had much at the time to justify it ; for those recent sponges 

 to which the fossil ones are chiefly allied were then unknown ; 

 the Lithistina and Hexactinellidse had first to be discovered before 

 the fossil forms could be said to belong to the same groups as the 

 known sponges of existing seas. The great mistake of D'Orbigny 

 lay in his assigning an original calcareous composition to the 

 skeleton of all the fossil sponges, of which the skeleton itself fur- 

 nishes no proof, since it exists in very various mineral states, and 

 since even when it is actually calcareous, is so, as we now know, as 

 a result of mineral replacement (see page 250). 



The recent discovery of the Hexactinellidse, especially of the Vitreo- 

 hexactinellidaB, and, above all, of the living species Myliusia Grayi 

 (the skeleton of which agrees essentially with that of Toulmin 

 Smith's Ventriculites), has given us the means of proving that the 

 distinction between the fossil and living sponges does not hold for one 

 of the largest of the fossil groups, that, namely, which is charac- 



* These and some fossil Saprolegneous organisms, common in the "copro- 

 lites" of the Cambridge bed, will form the subject of a separate paper. 



