164 Prof. W. J. Sollas on the 



at its distal end. In the Lithistidas the distinction into shaft 

 and rays is not strictly maintained, but all four rays liave the 

 same value, and start at once from a common centre ; moreover 

 they are the chief skeletal spicules, while in many of the 

 Choristidffi tlie mass of the chief spicules are uniaxial. 



Tlius, as the Tetractinellid character is the most funda- 

 mental, I propose to include all sponges which exhibit it as 

 Tetractinellida ; and as the next differentiation would appear 

 to result from the development of quadiradiate body-spicules 

 in Totractinellids wliich previously possessed only trifid super- 

 ficial spicules, I divide the Tetractinellida into TetractinelHda 

 Externa and Corapleta. The Externa Avill then fall into 

 Corticata and Non-corticata (Leptochrota, thin-skinned), the 

 Completa into Scolopidse (sharp-pointed), such as Dercihcs, 

 and Lithistida3. The term Choristid may still be con- 

 veniently used to designate Tetractinellids in which the 

 spicules are not conjoined into a lithistid network. 



The following Table shows the relations between tlie 

 different Tetractinellida as \ conceive them to exist :— 



Ijitliistidse 



Scolopidas Completa 



I Corticata 



Tetractixellida, Externa Leptochrota 



I 

 Drsmacidina. 



(ft 1 , Lithistidae 



I L ompleta ■ o i • i 

 ,p , ,. IT , ) ^ beolopian3 



retractinellida < n,„.f;io+n 



/ Externa 



I Corticata |. Choristid^. 

 ( Leptochrota j 



EXPLANATION OF THE PLATES. 



Plate VI. 



Pachymatisma Johnstoni. 



Fi(/. 1. Section througli the oscuhir tube (T), separated by the sphincter 



from the underlying oscular chamber (C): b, bacillar layer; 



V, vacuolated connective tissue ; c, gelatinous connective tissue ; 



f, fibrous layer. X 11. 

 Fit/. 2. Section through the sponge, showing the oscular tube («) and the 



oscular chamber {b) , -with main excnrrent canals (c) opening into 



it. Nat. size. 

 Fi(/. 3. Section through the commencement of an incuiTent tube, showing 



the poral canals, ectochone, and sphincter. X 15. 

 Fi(/. 4. View from above of the p(n'iferous r<iof : a, chone ; p, pore; (/, 



globate spicule. X 23. 



