REPOET ON THE TETEACTINELLIDA. 
cxix 
the Trisenosa and Ehabdosa ; in the former trisene spicules are present, but not in the 
latter. The following is a table of the proposed groups : — 
Order II. LITHISTIDA. 
Suborder I. HOPLOPHOEA. 
Demus I. Trlejstosa. 
Family I. Tetracladidse (Tetracladina, Zittel). 
Family II. Corallistidse (Rhizomorina, Zittel). 
Family III. Pleromidse (Megamorina, Zittel). 
Demus II. Rhabdosa. 
Family I. Neopeltidse. Family II. Scleritodermidse. 
Family III. Cladopeltidse. 
Suborder II. ANOPLIA. 
Family I. Azoricidse (Rhizomorina, Zittel). 
Family II. Anomocladidse (Anomocladina, Zittel). 
Relations of the Lithistida to the Choristida. — The presence of the desma through- 
out the Lithistida suggests a monophyletic origin for the order, for it is extremely 
improbable that this form of spicule has been separately evolved more than once. The 
resemblance of the Triaenose Lithistids to the Choristid Streptastrosa is so marked that 
there can be little doubt of the close phylogenetic connection of the two groups, and 
further, the fact that the desma commences in the Tetracladidse as a microcalthrops proves 
that the order of descent has been from the Choristida to the Lithistida, and not in the 
reverse direction. Again, since the point of connection with the Choristida is furnished 
by the Trisenosa, it is this which must be regarded as the ancestral demus ; the 
Rhabdosa are descended from it, and from the Rhabdosa the Anoplia. 
The adherence of the Lithistida to the Choristida being admittedly through the demi 
Trisenosa and Streptastrosa, the next question that arises is as to which of the families 
of these demi stand in closest connection, and I think it will be found that these are the 
PachastreUidse on the one hand, and the Tetracladidse on the other, for if we compare 
such a Pachastrellid as Pachastrella ahyssi with such a Tetracladid as Macandrewia we 
shall find a closer correspondence between the complements of spicules of the two, than in 
the case of any other two members of the two demi ; thus each possesses as microscleres, 
microxeas, microstrongyles, and spirasters, of similar, indeed almost identical characters. 
Both possess oxeas, and the calthrops of Pachastrella are represented by the tetracrepid 
