FROM THE LOWER CALCAREOUS GRIT OF YORKSHIRE. 55 



remarks that the acerates and the anehor-spicules appertaining to 

 the same sponges must have been equally abundant, affording a 

 copious supply of organic silica, which has been one source of the 

 chalcedonic matter so largely pervading portions of the Hag. 



Dr. Sorby again mentions the small reniform shells in his pre- 

 sidential address to the Society in 1879 *, and states that in the 

 Pema-hed in Dorsetshire and in certain beds in Yorkshire they 

 constitute as large a part of the bulk of the rock as the Forami- 

 nifera do in all but a very few exceptional specimens of Chalk. 



In 1880 t, Prof. Sollas compares these bodies with the globate 

 spicules of Geodian sponges in the Upper Chalk, and on the suppo- 

 sition that they belonged to similar sponges, changed Blake's name 

 to Geodites Sorbi/ana. 



The view that these detached reniform spicules, or globates as 

 they have been termed, belonged to the dermal crust of siliceous 

 sponges like those of the living genus Geodia, has been very gener- 

 ally accepted as correct, though an objection of considerable weight 

 could be urged against it, namely, that they occurred almost exclu- 

 sively in these rocks, without admixture with other forms of spicules, 

 whereas in the existing Geodian sponges the globates of the crust 

 form but a small proportion of the entire mass of the body-spicules, 

 and as these latter are larger and more robust than the globates, it 

 is natural to suppose that they would be present in the same rocks 

 with them ; and, in fact, this is the case in the contents of the 

 Upper Chalk flints of this country and in the Kreidemergel of 

 Westphalia, where the fusiform acerates and characteristic trifid or 

 fork-spicules of Geodia are mingled with the globates. The absence 

 of the detached larger acerate and fork-spicules in the Calcareous 

 Grit would tend to show that the globate spicules did not form the 

 orust of sponges like the recent Geodia, and this is now proved by 

 the occurrence of several more or less perfect specimens of sponges in 

 the same rocks with the detached spicules, which seem to be 

 entirely composed of this one form of globate spicule, and thus 

 explain its exclusive occurrence in the rock. Sponges with skeletons 

 thus formed are very distinct from any others yet discovered as 

 fossil, though there is a living gonus to which they may possibly be 

 related. Independently of their remarkable structural features, 

 these sponges are very exceptional instances of the preservation as 

 fossil of the connected skeleton, although its constituent spicules are 

 not organically united together, and the fact of their existence in 

 such great numbers that their microscopic detached spicules con- 

 tribute no small part of the mass of certain beds of rock, adds 

 further interest to their description. 



I may premise that the sponges in question belong in part to the 

 Natural History Museum at Scarborough, and in ])art to that at 

 York, and I am indebted to the kindness of Mr. C. ¥ox Strangways, 

 F.G.S., and to the Trustees of the York Museum, through Mr. H. 



* Proc. Geol. Sop. 1879, p. Al, — Appendix, p\. vi. fig. 1. 

 t Ann. & Mag. Nut. Hist. ser. 5, vol. vi. p. ;iy2. 



