so-called "Farringdon Sponges. 11 25 



be composed of a variable number of layers to which the same 

 observations apply, with the exception that the outlines in 

 the section show that they were much more irregular in form 

 (figs. 6-12) and themselves here and there threw out a spur 

 (fig. 8, b) y while in some instances the surface of the outer 

 layer was evidently tubercled (fig. 6, b). Hence the fibre 

 might be tuberculated as well as spurred here and there. Also 

 in one part of the slice there are two long straight rays like 

 those of an existing calcispicule, which, but for their point of 

 union and divergence being close to the edge, and therefore 

 broken away, would probably have been accompanied by a 

 third if not a fourth arm (fig. 11) ; and these evidently form 

 the axis of the sheath, which on one arm is continuous with 

 that of the neighbouring mesh (fig. 11, c) j so that we may 

 fairly infer that the colossal spicule of the axis of the fibre 

 throughout was a tri- or quadriradiate, although contorted 

 (we might almost say distorted) and modified in the form of its 

 arms to meet that of the meshwork. 



Although the crenulated structure immediately surrounding 

 the colossal spicule in Sestrostomella rugosa 7 to which I have 

 above alluded, is not so striking in Scyphia cylindrica, still it 

 has occurred to me, from observing an intermediate condition 

 in my specimen of Sestrostomella from the Jura, that it might 

 in like manner be the result of a section of the crenulated 

 laminae of the sheath, which, to a certain extent, might also 

 have been influenced by the form of the original spicule in the 

 first place ; still I observe in some fossil spicules of a Lithistid 

 from the Upper Greensand that have been mounted in Canada 

 balsam the straight lines of a shaft internally, while the outside 

 is tuberculated, illustrating what I have long since stated, 

 viz. that the ornamentations of a spicule are put on last upon 

 an originally plain shaft — that is, simply that the spicule 

 begins in this form and may end in a complicated one. 

 In some parts, where the fibre is in good preservation and 

 therefore solid, a transverse section shows that the axial 

 spicule was composed also of concentric layers (fig. 10, a), 

 which often seem to merge into the subsequent layers of the 

 sheath. ■ The latter, however, must not be considered homo- 

 logous with the sheath in Verticillites &c, where it is simply 

 a fossil adjunct, while in the genus Sestrostomella &c. it 

 appears to have been part of the original structure, however 

 much it may have become altered by subsequent fossilization. 



In no instance have I been 



able to see, in Scyphia cylin- 

 " pitchfork-like suicides " pre- 



drica, the free triradiates and u pitchfork-like spicules " J 

 sent in the fibre of Sestrostomella. 



Thus we have an extinct structure which, curiously enough, 



