27 



sents characters sufficient to distinguish it from the large 

 trifids of other species. Dr. Bowerbank also figures a single 

 large trifid as one of the characteristic spicules of Geodia 

 Zetlandica (Monogr. Brit. Spong. Vol. Ill, p. 15, PL 7, fig. 6). 

 On the other hand O. Schmidt in his descriptions of species 

 of Geodia and Stelletta from the Adriatic (Spong. d. Adriat. 

 Meeres 1862, p. 46, 49, 50, Taf. Ill, IV) represents two or 

 three forms of large trifid spicules (Anker) in the same species 

 and some of these appear common to two or more different 

 species. It thus becomes very uncertain whether the numerous 

 and very variable forms of large trifid spicules which are pre- 

 sent in this flint-meal should be taken to indicate as many 

 species of sponge, or whether two or three of different forms 

 may have belonged to one and the same species. Even 

 however, if we accept this latter view, it will be seen from 

 the number of forms of these spicules , that a considerable 

 number of species of this group lived together in the Cretaceous 

 ocean. Before describing these trifid spicules in detail, I pro- 

 pose first to refer to the aceralte spicules which probably 

 accompanied them. 



Acerate Spicules of Geodia and allied Genera. 



(Plate I, figs, i 3, 20, 21). 



Comparatively large, simple, straight and curved spicules, 

 which may be arranged under three different forms; First, 

 Straight spicules (fig. i) thickest in the centre and gradually 

 tapering towards the pointed extremities ; Second , Curved 

 spicules (fig. 2) also pointed at both ends, and thicker in pro- 

 portion to their length than the straight specimens ; Third, 

 Spicules of a rod-like character (fig, 5) nearly cylindrical or 

 diminishing very gradually towards the extremities ; I have 

 found none of these latter retaining their ends perfect. Sur- 

 face of these spicules apparently smooth originally. In none 

 has the central canal been preserved. The shorter forms of 

 these spicules measure 3,37 mm. long by 0,045 mm. wide, 



