Lt. OG wee 
a tendency to bifurcate. I propose to designate this form of 
spicule «szmple or compound quadrifid» according as the arms 
are simple or bifurcate. 
In addition to these trifid and quadrifid spicules, all or 
nearly all the sponges of this order have acerate spicules, 
often of two dimensions in the same species, and many have 
a surface layer of minute globate and globo-stellate spicules 
or small discoidal bodies. 
All the different forms of spicules which have been re- 
cognized in existing sponges of this order; the globate, globo- 
stellate and discoid bodies forming the outer cortex; the trifid 
spicules of the «zone» supporting the crust; the acerate and 
quadrifid spicules of the «<dedy»; and the anchoring spicules, 
are represented in the Horstead flint meal, and if we may 
judge by the relative number of the spicules, the Tetractinel- 
lid sponges must have far exceded the contemporary Lithistid 
and Hexactinellid sponges of the Chalk. In existing sponges 
of this order the skeleton spicules are distinguished by being 
much more robust than those of other types, and the same 
characteristic is displayed by the Chalk spicules. 
As regards the acerate spicules which I regard as_be- 
longing to this group, there is no peculiarity by which those 
belonging to different genera can be distinguished, nor are 
the minute spheres, stellates and discs of much service either 
in this respect, but it is a question how far the different forms 
of trifid and quadrifid spicules, which compose the zone and 
body spicules of these sponges, may be representative of 
different species. According to Mr. Carter's descriptions of 
existing species of Geodia and Stelletta from the South coast 
of Devon, (An and Mag. Nat. Hist. S. 4, Vol. 7, p..9, 
Pl. IV) from the Atlantic, (Vol. 18, p. 397, Pl. XVI); and 
also from the gulf of Manaar, (S. 5, Vol. 6, p. 485, Pl. V, VJ), 
there are present in each species, in addition to the globate and 
stellate spicules, the large and small acerates, and the minute. 
anchor-shaped trifid, a single form of large trifid which pre- 
