26 



a tendency to bifurcate. I propose to designate this form of 

 spicule <tsimple 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 <?;/<? supporting the crust ; the acerate and 

 quadrifid spicules of the frcdy] 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, VI), 

 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- 



