SPONGES 



147 



^Dercitus, Gray [Cret.] ; Corticella, Soil. ; Rhachella, Soil. ; Thrombus, 

 Soil. ; Samus, Gray ; *Ditriaenella, Hinde and Holmes [Eoc.]. 



SUB-ORDER 2. MICROSCLEROPHORA, SolL 



With tetraxon spicules of small size, comparable to microscleres. 



FAMILY 1. ICORTICIDAE, Vosm. With dense sarcenchymatous choano- 

 some and tough chondrenchymatous ectosome ; spicules microcalthrops 

 and candelabra, the latter localised at the surface of the body. Genus 

 ^Corticium, O.S. [Eoc.]. FAMILY 2. PLAKINIDAE, F.E.S. Choanosome of 

 loose, lacunar structure, collenchymatous ; the chondrenchymatous ectosome 

 scarcely or not at all oTeveloped ; spicules microcalthrops and their 

 derivatives, either by reduction (triactines, rhabdi). or by complication 

 (branching of the rays). Genera jPlakina, F.E.S. [loc.]$ Piacortis, F.E.S. ; 

 Plakinastrella, F.E.S. ; Plakinolopha, Tops. (Here Oscardla, finds its 

 nearest allies.) 



SUB-ORDER 3. OLIGOSILICINA, Vosm. 



Corticate sponges without tetraxon spicules ; siliceous skeleton reduced 

 to polyaxon microscleres (Chondrilla) or wanting entirely. FAMILY 

 CHONDROSIDAE, F.E.S. ; Genera Chondrosia, Ndo. ; Chondrilla, O.S. ; 

 Thymosia, Tops. 



ORDER 2. Tetractinellida (Marshall), Topsent, 1894. 

 Tetraxonida typically with triaene megascleres, or with desmas. 



SUB-ORDER 1. CHORISTIDA, Soil. 

 No desnias ; spicules never articulated to form a coherent skeleton. 



TRIBE 1. SIGMATOPHORA, Sollas. 



The microsclere when present is a sigmaspire. 



FAMILY 1. TETILLIDAE, Soil. With protriaenes, always present, and 

 sigmaspires, often wanting. Genera Tetilla, O.S. ; Chrotella, Soil., 

 Cinachyra, Soil. ; Craniella, O.S. ; Tethyopsilla, Ldf. 



TRIBE 2. ASTROPHORA, SolL 



One or more of the microscleres is an aster. 



Demus a Streptastrosa, SolL One of the microscleres is a spiraster 

 or, when this is not the case, one of the megascleres is a calthrops. ; 



FAMILY 2. THENEIDAE, SolL Megascleres, triaenes ; microscleres, 

 spirasters, and amphiasters ; the ectosome does not form a cortex ; ground 

 substance collenchymatous ; canal system eurypylous. Genus ^Thenea, 

 Gray [Cret.], (Fig. 24). FAMILY 3. fPACHASTRELLiDAE, Crtr. Megascleres, 

 calthrops, and rhabdi ; microscleres, spirasters, and microrhabdi. Genera 

 ^Pachastrella, O.S. [Garb. Cret. Eoc.] ; Calthropella, Soil. ; Characella, 

 SolL ; Poecillastra, SolL ; Sphinctrella, O.S. ; f Triptolemus, SolL [Eoc.]. 



Demus fi Euastrosa, Soil. Euasters always present, never spirasters 

 or sterrasters ; triaenes, but never calthrops amongst the megascleres. 



* Fossil forma. 



