PACHASTEELLA. 27 



Pachastrella plana, Hinde, n. sp. (Plate I. figs. 7, la.) 



The only example of this species is contained in the interior of a flint, and is a 

 fragment of the wall of a sponge which appears to have been palmate in form. The 

 wall itself is about 3 mm. in thickness, the margins are attenuated. It is composed 

 of spicules loosely disposed over each other without being attached together. The 

 spicules appear for the most part to be only three-rayed ; occasionally, however, a 

 fourth ray is represented by a very short process. The rays of the spicules are 

 straight or slightly arched, nearly in a horizontal plane, and they spring from the 

 centre at nearly equal angles. Two of the three horizontal arms are not infrequently 

 longer than the third. The arms or rays are nearly cylindrical, their ends apparently 

 rounded. The arms vary from 0'6 mm. to nearly 1 mm. in length. The spicules are 

 arranged parallel to the surface of the sponge-wall, so that there are small inter- 

 spaces between the rays. 



The form and arrangement of the spicules readily distinguish this from the other 

 described species of Pachastrella. Similar spicules to those composing the walls 

 of this species are present detached in the Horstead flint. Foss. Sp. Sp. p. 48, 

 t. iii. f. 27. 



Distribution. Upper Chalk : Upware, Cambridgeshire. 



Fragments of Flint, Chert, and other Rocks composed of Sj)onge-spicules. 



(Plate I. figs. 10, 11, 12.) 



Flint or chert from the Portland Oolite of Upware near Weymouth, containing 

 trifid spicules. Fig. 10 is a magnified representation of a thin microscopic section. 



Thin bands of rock from 25 to 75 mm. each in thickness, from the Lower Green 

 Sand near Hasleraere in Surrey, are nearly entirely composed of sponge-spicules, 

 which form a compact mass now cemented together in a siliceous matrix. When 

 exposed to the weather the spicules stand out distinctly from the matrix. The 

 spicules are mostly acerates and trifids ; a few forms of Lithistid spicules are also 

 present. Fig. 11 is a magnified representation of a minute portion of the weathered 

 surface of one of these sponge-beds. Nodules of rock from the Lower Green Sand 

 of Folkestone and from Badbury Hill near Farringdon are similarly made up of 

 spicules. 



Bands of chert in the Upper Green Sands near Ventnor, Isle of Wight, are filled 

 with spicules, which, like those from the Lower Green Sand, are principally acerates 

 and trifids. Fig. 12 is a representation of a thin microscopic section of this chert. 

 Beds and layers of spicules also occur in strata of this age at Blackdown, and in the 

 Haldon Hills near Exeter. 



Flints from the Upper Chalk sometimes exhibit spicules imbedded in the siliceous 

 mass ; but more frequently the spicules occur infilling cavities in the interior of the 



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