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. III, 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. III, 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 acerate 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. 1) 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, 
