434 Mr. H. J. Carter on the so-called 



best of my knowledge, is so far peculiar that it is never seen 

 in a siliceous state, any more than the form of the simple ace- 

 rate of a Renierid or Chalina is observed in a calcareous one 

 among the recent sponges. 



I then thought, if I could get an instance such as those 

 mentioned by Prof. Zittel and Mr. Sollas respectively, of a 

 siliceous sponge presenting itself under a calcareous fossilized 

 form, I might then see how far its fibre would represent that 

 of the Farringdon sponges ; and it so happens that there 

 exist among my Farringdon collection two cylindrical fossils 

 each about one inch and a half long by half an inch broad, 

 with an axial cavity and furrowed oral aperture, so like 

 Lithistids that if they had not presented the same kind of 

 white, round, open, reticulated, calcareous fibre on the sur- 

 face as that of the Farringdon sponges with which they 

 were associated, I should have set them down immediately 

 as such. But on making polished longitudinal and trans- 

 verse sections of the bodies of these fossils for further ob- 

 servation (by reflected light), I found that their spiculation 

 was Lithistid, and that instead of this spiculation presenting 

 itself under the form of continuous reticulated fibre of the 

 same size, like that to which I have alluded (that is, wrapt 

 up in calcareous material), the sjricides were naked, and, in 

 this condition, bore unmistakable evidence of that Lithistid 

 nature which, if viewed while invested in the calcareous fibre 

 of the surface so characteristic of the so-called Farringdon 

 sponge when seen under the microscope by transmitted light 

 in a slice ground down to extreme thinness, would lead to the 

 difficulty above expressed, the uncertainty of whether we were 

 looking at the remains of an originally siliceous or calcareous 

 spiculation. 



Thus my anticipation was realized, and some at least of 

 the Farringdon sponges proved to have been originally 

 siliceous. 



In reducing the slice of Elasmostoma from my Farringdon 

 sponges to a sufficient thinness for microscopical examination 

 it proved so intractable, from the presence of mineral crystals 

 much harder than my grinding-stones, that I was obliged 

 to have recourse to the Neocomian specimen for this purpose, 

 kindly sent to me by Prof. Zittel ; but just previously to the 

 breaking-up of the slice of the Farringdon one, although not 

 until it had been almost sufficiently reduced, I saw that it was 

 in structure similarly composed ; nor could I help remarking 

 at the same time the resemblance, in form at least, that 

 existed between these two specimens and the Lithistid Ver- 

 rucidina auriformis, also represented by Zittel (Handbuch &c. 



