Nodules of the Trimmingham Chalk. 389 



Lithistid, though Thenea muricata and Stelletta discophora, as 

 well as other sponges, contain very similar forms. Amongst 

 recent Lithistids we find it in Corallistes microtuberculatus 

 and C. Botverbankii, among fossil forms in Callopegma and 

 Turonia, Pachinion and Scytalia. The dermal spicule of 

 Pachinion and Scytalia differs, however, in possessing shorter 

 and less slender arms ; while the dermal spicules of the other 

 two genera, though much more like our forms, are associated 

 with Tetracladine skeletal corpuscles, and these are scarcely 

 plentiful enough in our deposit to account for the large 

 number of dermal spicules which occur in it. On the other 

 hand, Rhizomorine corpuscles are more abundant, and, consi- 

 dering their resemblance to the corpuscles of Corallistes, may 

 very well have been associated with dermal spicules similar 

 to those which are known to exist in this genus. Thus one 

 may group the dermal spicule PI. XIX. fig. 4 with the skele- 

 tal corpuscles of figs. 5 and 12. The characters of such a 

 group would approach those of Scytalia ; and so we might 

 venture to add to it the uniaxial forms PI. XIX. figs. 7 

 and 9, which resemble those figured by Zittel as occurring 

 in Scytalia turbinata, and which are common in the flint 

 nodules. 



Fig. 6 should probably not be included here, as it is much 

 more likely to have belonged to some Echinonematous sponge. 

 There are many spicules in the flints similar to fig. 6, but 

 much more abruptly bent ; and I fancy they must have be- 

 longed to a sponge like Dictyocylindrus. 



Macandrewites Vicaryi. (PI. XIX. fig. 20.) 



Dactylocalycites Vicaryi, Carter, Fossil Sponge-spicules, loc. cit. pi. vii. 

 figs. 1, 2, 6 ; Zittel, Coeloptychium, Taf. vii. fig. 31 ? 



This form is remarkably similar to the dermal spicule of 

 Macandrewia clavatella, 0. S., a slight difference in size (this 

 being the larger) chiefly distinguishing them. The skeletal 

 corpuscle shown in fig. 16 is likewise similar to that of Mac- 

 andrewia ; and we place the two together under the name of 

 Macandrewites Vicaryi. 



CORALL1STITES? (PI. XIX. fig. 27.) 



This is a trifid spicule with branching arms, tuberculated 

 on the upper surface and prolonged into a long shaft below. 

 It differs apparently from the dermal spicule of Corallistes 

 nolitangere in bearing tubercles on the upper surface of its 

 rays ; but in other respects it is very similar. 



