FROM THE LOWER CALCAREOUS GRIT OE YORKSHIRE. 57 



transpai'ent that little more than their outlines can be distinguished ; 

 whereas in glycerine they stand out prominently, and their surface 

 features are clearly seen. The spicules vary from ellipsoidal to 

 subspherical in form, with a small notch or hilum which gives 

 them their characteristic kidney-shaped appearance. In sections 

 of the sponge-wall they range from '11 to '15 millim. in diameter, 

 hut some of the detached terms are not more tlian -08 millim. 

 in thickness. On the surface of the best-preserved spicules there 

 is an ornamentation of minute subcircular spots w^ith shaded 

 borders, aj)parently regularly quincuncially arranged ; the spots are 

 very minute, averaging only -002 millim. in width (hgs. 6 & 7). They 

 were first noticed by Prof. Blake, and supposed by him to indicate 

 perforations in a surface-shell. I have also noticed in a few of the 

 spicules traces of very fine lines or fibres radiating from their centres 

 to the surface (fig. 8). These and the surface-markings indubitably 

 prove the structural similarity of the fossil spicules to the siliceous 

 globates of the crust of the recent Geodia and other sponges ; for it 

 has long been known that each of these recent globate spicules is built 

 up of a great number of minute siliceous radial fibres which extend 

 from the centre of the spicule and terminate on its outer surface in 

 nodose or spined ends, and the traces of radiate fibres and the surface- 

 spots in the fossil spicules are really due to the original structure of 

 minute fibres like those of the globates of the recent Geodia. 

 The subcircular spots representing the terminal ends of the fibres 

 in the fossil globates are, however, much finer than those of recent 

 forms, but they correspond more nearly in this respect with the 

 globates of the recent genus Placospomia, mentioned below. 



Hitherto no sponge has been described, either fossil or recent, with 

 a skeleton entirely composed of globate spicules, as seems to be the 

 case Avith this fossil form, and its svstematic relations are conse- 

 quently somewhat uncertain. As a rule, globate spicules, like 

 those of this fossil, form a firm dermal crust to sponges whose main 

 skeletal spicules are of quite a dift'erent character ; thus in Geodia, 

 as already mentioned, the mass of the skeleton of the sponge usually 

 consists of relatively large fusiform acerates and long-shafted forks 

 and anchor spicules ; and in the Jurassic Lecanella 2^cf'terceformis, 

 Zittel, whilst the surface is stated by * v. Zittel to be covered with 

 innumerable globate spicules of precisely the same character and 

 size as in our fossil, the main skeletal si)icules are irregularly 

 branching Lithistid forms. There is, however, a very remarkable 

 recent genus, Placospomjia, Gray, in which the globate spicules form 

 a solid interior axis to the sponge, of much the same character as 

 the anastomosing plates of the interior of the present fossil. In the 

 type species of this genus, P. melohesioides, Gray t, which I have had 

 an opportunity of examining in the Natural History Museum, 

 South Kensington, there is in addition to the solid axis of globate 

 s])icules a dense dermal crust of the same kind of spicules, and in 



* '• Studieu liber fossile Spongien," Abh. der k. layer. Akad. dcr Wiss. CI. ii. 

 Bd. xiii. Ab. i. p. 13o. 



t Proc. Zool. Soc. Jan. 1867, p. 127. 



