60 DE. G. J. HINDE ON A NEW GENUS OF SILICEOUS SPONGES 



and their individual spicules are scattered over the sea-bottom. A 

 similar fate undoubtedly befell the great majority of these sponges 

 in the oceaa of the Lower Calcareous Grit ; for, whilst the entire 

 skeletons are very seldom met with, the detached compoiient spicules 

 form a very notable proportion of the rock-matrix in which they are 

 imbedded, and the specimens may be considered as the scantiest 

 relics of an enormous multitude, which by specially favourable cir- 

 cumstances have escaped disintegration. 



These sponges also furnish additional proof of the connexion be- 

 tween these organisms and beds of chert and silica (other than 

 quartz). Thus in the Cliff at Scarborough, as Mr. Hudleston has 

 shown, there is in the Lower Calcareous Grit, from which these sponger 

 have been obtained, a bed of 3 feet 4 inches (about 1 metre; of 

 intensely hard chert, and beneath this, 30 feet (9 metres) of a 

 calcareous grit, largely cemented by silica. There can be little 

 doubt that most of the silica in this considerable thickness of rock 

 (not reckoning the sand grains) is due to the siliceous sponges ; for 

 throughout these beds, not only at Scarborough itself but in the 

 adjoining areas, Mv. Hudleston has pointed out that where these 

 minute giobates are not themselves now present in the siliceous 

 strata, they are represented by minute empty moulds, which bear 

 witness that the spicules themselves have been dissolved and the 

 resulting silica has been redeposited to form the siliceous cement in 

 the rock. Mr. Hudleston has given so excellent a description of 

 the Calcareous Grit wliich has been thus formed that I venture to 

 quote the passage*: — " It is almost entirely devoid of calcic carbonate, 

 and consists chiefly of a poriferous mass of siliceous matter, which 

 includes a quantity of extremely fine quartzose sand. The cherty 

 portions exhibit the pores also, but then they seem further apart 

 and less connected. Sometimes the pores are empty — pin-hole 

 structure ; sometimes they contain a white powder, which is either 

 silica or a silicate — speckled structure. In a greater or less degree, 

 the above characters may be recognized in very much of these yel- 

 low sponge-cake calc.-grits, giving rise to the idea of the decompo- 

 sition of granular bodies of almost microscopic minuteness, which, 

 during the formation of the rock, had formed no inconsiderable 

 portions of its mass, but which, subsequent to consolidation, have 

 in most cases been removed by solution." Proc. Geologists' Asso- 

 ciation, vol. iv. (1875), p. 32, sep. cop. 



EXPLANATION OF PLATE VI. 



Bhaxella perforata, Hinde, gen. et sp. nov. 



Fig. L An imperfect subpalmate specimen, showing the irregular mode of 

 growth and the perforate character of the wall. Natural size. The 

 .specimen is now quite free from the matrix. It comes fiom Scar- 

 borougli, and now belongs to the Museum at York. 



2. A fragment of another specimen in which the wall exhibits a flattened 



trabecular structure with slit like apertures. Natural size. 



3. A transverse section of the above, showing the labyrinthic disposition of 



the laminee of the wall. Enlarged two diameters. 



* Proceedings of the Geol. Assoc, vol. iv. (1875), p. 44, sep. cop. 



