SPONGES 147 



iDercitus, Gray [Cret.] ; Corticella, Soil. ; Rhachella, Soil. ; lliromhus, 

 Soil. ; Sainus, Gray ; * Ditnacnclla, Hinde and Holmes [Eoc.]. 



Sub-Order 2. Microsclerophora, SolL 



With tetraxon spicules of small size, comparable to niicroscleres. 



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.]. Familt '2. Plakinidae, F.E.S. Choanosome of 

 loose, lacunar structure, collenchymatous; the chondrenchymatous ectosome 

 scarcely or not at all developed ; si)icules microcalthrops and their 

 derivatives, either by reduction (triactines, rliabdi) 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 sjionges -without tetraxon spicules ; siliceous skeleton reduced 

 to polyaxon microscleres (Chondrilla) or wanting entirely. Family — 

 Chondrosidae, F.E.S. ; Genera — Chondrosia, Ndo. ; Chondrilla, O.S. ; 

 Thymosin, Tojjs. 



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

 Tetraxonida typically with triaene megascleres, or with desmas. 



Sub-Order 1. Chouistida, 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 

 9igmasi)ires, often wanting. Genera — Tetilla, O.S. ; Chrotella, Soil., 

 CMiac%r«, Soil. ; Craniella, O.S.; Tethyopsilla, hdi. 



Tribe 2. Astrophora, SolL 



One or more of the microscleres is an aster. 



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

 ■ )r, when this is not the case, one of the megascleres is a calthrops. , 



Family 2. Theneidae, Soil. Megascleres, triaenes ; micro.scleres, 

 spirasters, and amphiasters ; the ectosome does not form a cortex ; gi'ound 

 substance collenchymatous; canal system eurypylous. Genus — jThenea, 

 Gray [Cret.], (Fig. 24 . Family 3. jPachastrellidae, Crtr. Megascleres, 

 calthrops, and rhabdi ; microscler&s, spirasters, and microrhalxli. Genera 

 — jPachastrella, O.S. [Carb. Cret. Eoc] ; Calthropella, Soil. ; Characella, 

 Soil. ; Poecillastra, Soil. ; Sphindrella, O.S. ; \ Triptolemus, Soil. [Eoc]. 



Demus /i — Euastrosa, Soil. Euasters always present, never spirasters 

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



* Fossil forms. 



