CIV 
THE VOYAGE OF H.M.S. CHALLENGEE. 
Family III. Corticidse. — The chamber-system is aphodal. The spicules 
are candelabra, or amphitrisenes, or trichotria3nes, or trisenes 
with a spinose surface. 
Suborder II. Trisenina. — The cladomes of the fully grown trisenes are situated 
in the ectosome. 
Family I. Tetillidse. — The chamber-system is eurypylous or aphodal. 
The microscleres are sigmaspires or toxaspires. The form of the 
protrisene is characteristic. 
Family II. Theneidse. — The microsclere is a spiraster. The chamber- 
system is eurypylous. 
Family III. Stellettidse. — The characteristic microsclere is the aster. 
The chamber-system is aphodal. 
Family IV. Geodinidse. — The characteristic spicule is the sterraster. 
The chamber-system is aphodal. 
The distinction between the Trisenina and the Tetradina cannot very well be 
maintained, at all events without a redistribution of their contents ; for the Tetradine 
family Pachastrellidse is connected with the Trisenina by the family Theneidse ; indeed, 
there is a most evident transition from Thenea to Pachastrella through the inter- 
mediate genus Pcecillastra [Normania). If the suborders Trisenina and Tetradina are 
to be retained, it can only be by transferring the Pachastrellidse to the Trisenina, and 
then the names for the suborders become inappropriate. And this was already the case 
owing to the presence of trisenes in the Corticidse. I therefore abandon these suborders, 
and now proceed to discuss the facts and principles which may lead us to a natural 
classification of the Choristida. We commence with a digression, for as in con- 
sidering the best way to represent a tree, which of all natural objects best serves to 
represent the nature of zoological classification, we begin with the trunk and pass on to 
the branches and leaves, so in the present case we have commenced with the higher 
groups and are proceeding down to the lower ; but this plan is not that of strict logic, 
and amongst its other inconveniences is that of the present digression, which it involves. 
And first we are met by the old time difficulty as to what constitutes a species. Is it a 
collection of similar individuals, separated from others by sexual sterility and the absence 
of intermediate forms ? That it is a collection of similar individuals, united by the 
possession of similar characters and distinguished from others by the presence of 
common differences, we shall admit, but for my part I refuse altogether to have any- 
thing to say to the two additional limitations ; good species may exist and yet be united 
on all sides by transitional forms, and as to mutual fertility, one would inquire how this 
test in the majority of cases is to be applied, and in the next, of what value in a 
morphological classification such a distinction is when it has been ascertained ; it would 
