62 



Dermal spicules of Lithistids. 



(Plate III, figs, ir, 12, 15). 



Compound trifid spicules with relatively short, straight 

 or slightly curved, pointed shaft and with spreading head- 

 rays. There are three forms of these spicules, small in com- 

 parison with the trifid spicules belonging to Geodia, and 

 probably belonging to different species of sponge. The first 

 (fig. 1 1 ) has the shaft curved and the primary head rays 

 also curved and directed upwards; they bifurcate into very 

 small pointed secondary rays. An average spicule is 0,67 5 mm. 

 in length and the thickness of the shaft 0,056 mm. In the 

 second form (fig. 12) the shaft is straight and pointed, the 

 primary head rays are very long in proportion to the shaft 

 and directed forward and the secondary rays are similarly 

 long, divergent, and projecting upwards. Total length of the 

 specimen figured 0,787 mm. diameter of shaft 0,045 mm - '> 

 width across head rays 0,675 mm. This form has been found 

 also in the Irish Chalk, (Wright: op. cit. PL II, fig. 16). The 

 third form (fig. 1 5) has also a straight shaft but the rays are 

 very short and pointed, and they bifurcate so close to the 

 top of the shaft that the primary ray can scarcely be distin- 

 guished. The length of the specimen figured is 0,99 mm. 

 thickness of shaft 0,056 mm. 



As regards their dimensions, these spicules correspond 

 much closer with the trifid spicules which are present as 

 surface spicules in different genera of Lithistid sponges, than 

 with the trifid zone spicules of Geodia, and it seems to be 

 very probable, that they are allied to the surface spicules of 

 Cor alii stes, O. Schmidt. 



Dermal? spicule of Lithistid? 



(Plate I, figs. 29, 30) 



Spicule consisting of a slender, more or less elongated, 

 nearly cylindrical shaft, having at one extremity four elongated 



