REPORT ON THE TETRACTINELLIDA. 
ciii 
established as a natural group, our succeeding account will have reference to its 
subdivision into smaller groups; the first to attempt this was Vosmaer,^ who proposed 
the following classification : — 
Suborder Tetractina. 
Family I. Geodidse (Genera — Geodia, Isops, Synops, Pachymatisma, Cydonium, 
Caminus). 
Family II. Ancorinidse (Genera — Stelletta, Papyrula, Ecionema, Thenea, Dercitus, 
Agardiella, Sphinctrella, Ancorina, Trihrachion, Tethyopsis, Tricentrium, Ophiraphi- 
dites, f Craniella, Tetilla). 
Family III. Placinidse (Genera — Placina, Placortis, Placinastrella). 
Family IV. Corticidse (Genus — Corticium). 
The adoption of Schulze’s family Placinidse, and the addition of the Corticidse, I 
recognise as necessary, but it is hard to see on what grounds Schmidt’s two families, 
the Geodiidse and Ancorinidse, are retained, either both should have been merged into 
one, or the Ancorinidse should have been broken up and its members redistributed ; for 
the Geodiidse, although they are conveniently regarded as a natural family, are 
distinguished from the Stellettidse by a single character only, the possession of the 
sterraster, and these two groups are much more closely allied together than the 
members of the Ancorinidse are to each other ; but what makes the retention of this 
family all the more inexplicable is the fact that Carter^ long ago recognised its 
unnatural character, and separated from it the Tetillidse, Theneidse, and Pachastrellidse, 
as groups of equivalent value to the Geodiidse. It is true that Carter did not call these 
groups families, but subfamihes, but this is a point of no consequence, any more than 
the fact that to the Tetillidse Carter gave the name Tethyina. 
In my preliminary account on the Challenger Tetractinellida I proposed a different 
arrangement ® to that which is now adopted in this Report. The following is a short 
summary of that classification ; — 
Order I. Choristida. 
Suborder I. Tetradina. — The characteristic spicules are calthrops, candelabra, 
amphitrisenes, or modified trisenes. 
Family I. Placinidse. — The chamber-system is eurypylous. Candelabra 
are present. 
Family II. Pachastrellidse. — The chamber-system is either eurypylous 
or aphodal. The spicules are simple calthrops. 
1 Vosmaer, Bronn’s Klass. u. Ord. d. Thierreichs, Porifera. 
* Carter, Ann. and Mag. Nat. Hist., ser. 4, vol. xvi., 1875 ; ser. 5, vol. xi. p. 362, 1883. 
3 SoUas, Sci. Proc. Roy. Dull. Soc., N.S., vol. v. p. 177, 1886. 
