386 Mr. W. J. Sollas on the Flint 



rostrum of a Belemnitella ; but none of these included bodies 

 have determined the general form of the nodule in which they 

 occur. The surface of the passages in the flint bristles with 

 large spicules having one end imbedded in the flint and the 

 other projecting freely outwards ; these spicules may serve 

 to determine whether the chalk or the flint of the nodule 

 occupies the place of the sponge which, we believe, determined 

 by its existence the formation of the flint. If the chalk, then 

 the points, if the flint the heads of the spicules should be seen 

 projecting into the cavity. On examination one finds both 

 heads and points projecting, but the heads least frequently, 

 perhaps because they have been broken off. These observa- 

 tions go but a very little way towards proving any thing ; by 

 analogy with other flints, however, we should conclude that 

 the chalk represents the original sponge ; and so far as the 

 position of the spicules in the nodules indicates any thing it is 

 in this direction. 



We now proceed to the determination of the spicules, com- 

 mencing with those belonging to the Lithistidee. It will be 

 seen that the Lithistidee are placed within the Tetractinellidse 

 as a suborder. This necessitates the formation of a new name 

 for those Tetractinellidaj which are not Lithistids ; and we pro- 

 pose to call them Choristidae, since their spicules are separate 

 and not locked together into a network. Thus, just as we 

 have Dictyonine and Lyssakine Hexactinellidas, so we can 

 speak now of Lithistid and Choristid Tetractinellidge. 



Tetractinellidse. 



Lithistidse. 



Tetba cladina . 



Discodermites cretaceus. (PI. XIX. fig. 1.) 



Dttctylocaly cites polydiscus, Carter, Ann. & Mag. jSat. Hist. ser. 4, 

 vol. vii. p. 122, pi. vii. figs. 3-5. 



This spicule, with its nearly circular disciform head and 

 conical shaft, and those of fig. 2, with their smooth cylin- 

 drical arms and botryoidal apophyses, precisely resemble the 

 dermal spicule and skeletal corpuscle, respectively, of Disco- 

 dermia polydiscus, Bocage ; they may therefore be regarded 

 as having belonged to that or a very closely allied sponge. 

 To identify them specifically with it, in the absence of fuller 

 information, would perhaps be going too far ; and hence it 

 may be preferable to ascribe them to a distinct species provi- 

 sionally. As the recent D. polydiscus possesses a long ace- 

 rate spicule, we may associate with the two forms already 



