Hexactinellidse and Lithistidge. 461 



finally, after the fully developed fibre has become deciduous, 

 their forms reappear in the state of moulds caused by a process 

 of absorption of the spicule not yet explained. 



2nd. That by the reappearance of the forms of the spicules 

 we are enabled to determine the species if previously known in 

 a living state, although nothing but the bare deciduous vitrified 

 structure may remain ; and therefore, where no living specimen 

 of the species has been found, to determine what kind of 

 spicules it originally possessed. 



3rd. That there are no grounds for stating that a hexacti- 

 nellid sponge exists in which the fibre is fistulous — that is, 

 pervaded by a continuous central canal, as in the keratose 

 sponge Luffaria (D. et M.) ; but, on the contrary, that the 

 vitreous fibre is always based on an axis of sexradiate 

 spicules. 



With the deciduous specimens of Aphrocallistes Bocagei 

 there has been no difficulty in determination, because we know 

 what the characteristic spicules of this sponge are in its living 

 state. 



But the case is not so satisfactory with Farreaocca^ of which 

 nothing but deciduous specimens have yet been found. How- 

 ever, here it is evident that, besides the common sexradiate 

 spicules of the skeleton (PI. XVI. fig. 4, hlh)^ there was the 

 spined one described as no. 2 under Aphrocallistes Bocagei 

 (tig. 5, Z*), and the scopuline shaft like that figured by Schmidt 

 in his Farrea occa (figs. 6,b & 7,b), to which I have before 

 alluded. 



Lastly, it might be stated respecting Farrea occa that, 

 although we know that, in addition to the common sexradiates, 

 it possessed the spined spicule no. 2 of Aphrocallistes Bocagei 

 and another like the scopuline spicule of Farrea facuiida, 

 while the latter spicule is by no means identical with that 

 figured by Schmidt (neither have we seen the spino-capitate 

 spicule nor the rosette which are also ngured by the same 

 author as characteristic of F.facunda)^ yet the field from which 

 we have obtained the facts above mentioned respecting F. occa 

 i^ very limited ; so that by-and-by, if a living specimen is not 

 found of F. occa, but still more deciduous ones, more of the 

 spicules it originally possessed may be made known after a 

 like manner. At the same time, it should be remembered 

 that the general figure (Taf. ii. fig. 10) , as Avell as the detail of 

 its skeleton which Schmidt has given of his F. facunda, are 

 so identical with Mr. Kent's, Mr. Gassiot's, and the deep-sea 

 ones of F. occa before mentioned that without a certain know- 

 ledge of all the isolated spicules of the latter it would be very 

 hazardous to state that /"'. occa and F.facunda were not one 



