the Greensand compared with those of existing Species. 115 



general plan on which I shall endeavour to show that many 

 of the spicules have been developed, than to identify some of 

 them with existing species. 



All the figures have been drawn on the same scale^ viz. l-24th 

 to l-1800th of an inch, whereby their relative sizes respectively 

 can be at once seen, their real sizes computed by compass and 

 rule, and the introduction of measurements avoided. 



Some of the figures appear very large ; but when it is re- 

 membered that others would have been inconveniently small 

 if the whole had been proportionally more reduced in size, this 

 win be fully explained. At the same time it should be re- 

 membered that, as all are sufficiently large to be extracted 

 with the aid of a simple but powerful lens, and therefore that 

 there are hardly any spicules present so small as to require 

 the microscope for detection, it is evident that nothing but 

 coarse and large spicules exist in this deposit, that if there 

 were originally minuter forms in it there is nothing now left 

 to show that this was the case, and, therefore, that the great 

 bulk of the sea-shore Spongiadaj, in which all the spicules are 

 too small to be seen individually with a quarter-inch lens, 

 have no representatives in this deposit. 



Of the deep-sea sponges, such as Hyalonema, Holtenia^ 

 Pheronema^ Askonema, Corhitella, Gray (?), &c., there is, of 

 course, no representative ; their delicate spicules slightly held 

 together by equally delicate sarcode, and their habitat in the 

 deep valleys of the ocean, almost entirely preclude the jiossi- 

 bility of their spicules ever reaching sucli tidal currents as 

 could drift together the gritty materials of the Haldon deposit. 



Not so, however, with the Coralliospongi^ of Dr. Gray, and 

 the Euplectellidffi, whose spicules are supported by a rigid 

 structure of keratose fibre silicified. The habitat of the latter, 

 at the Philippine Islands, in from ten to twenty-four fathoms 

 (Bowerbank, Proc. Zool. Soc. London, 1869, p. 344), and that 

 of most of those in the Gulf of Florida &c., forwarded by M. de 

 Pourtales to Dr. Schmidt for examination, in minimum depths 

 of from 90 to 152 fathoms (Grundziige einer Spong. Faun, des 

 atlantisch. Gebietes, 1870) show that these might have repre- 

 sentatives in such deposits ; and thus we find that, in the spi- 

 culiferous sand of Haldon Hill and Black Down, nearly half 

 the organic remains consist of spicules and fragments of the 

 silicified fibre of the Coralliospongiee. The rest, chiefly belong 

 to that group of sponges for which I have proposed the 

 term " Pachytragias " (Annals, vol. vii. Jan. 1871), viz. the 

 Geodidffi, Stelletta, Dercitus^ &c., but not the Tethyadaj proper, 

 of which T. cranium is the type, since the spicules of these 

 sponges, with the exception, perhaps, of the large acerate one, 



