REPORT ON THE TETRACTINELLIDA. 
317 
The resemblance of this sponge to Scleritoderma 'packardi is very close, the 
characters of the pores and oscules are similar, and the same kinds of spicules are present 
in both ; the specific distinction is well marked, resting not only on the difference in 
external form, though this is considerable, but more particularly on the dimensions of 
the microstrongyle, which is twice the length in Scleritoderma packardi of that in 
Scleritoderma Jlabelliformis, 
Family II. Cladopeltida:. 
Rhabdosa in which the ectosomal spicule is a desma highly branched in a plane 
parallel to the surface. Microscleres are absent. 
Genus 1. Siphonidium, 0. Schmidt. 
The oscules are the simple terminations of narrow external tubular processes. 
Siphonidium cap>itatum, n. sp. (PL XXXVII.). 
Sponge (PI. XXXVII. fig. 1). — Small, growing from an attached incrusting base into 
variously shaped lobes, from the sides and ends of which slender, straight or crooked 
tubes with a terminal aperture or oscule are produced. The axial canal of the tubes 
is continued without any apparent change of dimensions a considerable distance into the 
lobes, within which it terminates by breaking up into branches. 
The surface is covered by a smooth, thin, imperforate, wrinkled skin, the wrinkles of 
which are accurately reproduced by the surface of the skeleton below. Pores (?). 
Spicules. — I. Megascleres. 1. Ectosomal desma (PI. XXXVII. fig. 2), a broad undu- 
lating epirabd, giving off branches from the convex sides of the curves, and bifurcating 
at the ends ; the cladi subdivide and give off lateral branches, and all the branches finally 
terminate in twig-like processes. The desma is depressed and ramified in a plane parallel 
to the surface, and the angles between the branches are mostly well rounded off ; the 
twig-like endings of one desma are overlapped by those of its neighbours, and thus a 
close-meshed superficial network is produced without actual zygosis. 
2. Choanosomal desmas (PI. XXXVII. figs. 3-8), the characters of these are best 
conveyed by the illustrations. Forms like figs. 4-8 are richly developed in the tubular 
processes. Forms like fig. 3 occur beneath the ectosomal desmas, or immediately beneath 
the skin where these are absent. They present a straight epirabd, from the outer surface 
of which spined processes are given off, which end at the general surface of the sponge ; 
from the inner face proceed a number of cladi (sometimes as many as seven), which 
