SPONGES 147 



jDercitus, Gray [Cret] ; Corticella, Soil. ; Bhachella, Soil. ; Thrcrmbus, 

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



SUB-ORDER 2. MICROSCLEROPHORA, Soil 



With tetraxon spicules of small size, comparable to microscleres. 



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

 some and tough chondrenchymatous ectosome ; spicules microcalthrops 

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

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

 loose, lacunar structure, colleiichymatous ; the chondrenchymatous ectosome 

 scarcely or not at all developed ; spicules microcalthrops and their 

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

 (branching of the rays). Genera jPlakina, F.E.S. [Eoc.] ; Placortis, F.E.S. ; 

 Plakinastrella, F.E.S. ; Plakinolopha, Tops. (Here Oscarella 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 desmas ; 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, Soil 



One or more of the microscleres is an aster. 



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

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



FAMILY 2. THEXEIDAE, Soil. Megascleres, triaenes ; microscleres, 

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

 substance colleiichymatous ; canal system eurypylous. Genus jThenea, 

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

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

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

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



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

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



* Fossil forms. 



