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THE VOYAGE OF H.M.S. CHALLENGER. 
Synthecium alternans, n. sp. (PL XXXVII. figs. 2, 2a). 
Trophosome. — Stem unbranched, fascicled towards the base, set with pinnately 
disposed, equidistant, alternate ramuli ; ramuli divided into equal internodes, each of 
which carries a hydrotheca near its distal end. Hydrothecse alternate, tubular, deep, 
adnate to the internode for about half their height, then diverging at a high angle and 
terminating in a circular and entire orifice. 
Gonosome. — Gonangia oval, compressed, destitute of annulation, and with a 
terminal, scarcely elevated orifice. 
Locality. — Off Port Jackson ; depth, 30 to 35 fathoms. 
i 
The present species is of great interest as affording an instance of the characteristic 
synthecial structure, with an alternate disposition, of the hydrothecse. It is a strong, 
rather rigid form, with the stem fascicled towards the base, but becoming monosiphonic 
distally, and attaining a height of between two and three inches. 
The gonangia are compressed so as to present a lenticular form, more convex on 
one side than on the other, the more convex side being that which is turned towards 
the supporting pinna. They are entirely destitute of all trace of annulation and have 
their walls perfectly smooth. The male gonangium is smaller than the female, but 
otherwise differs but little from it in shape. 
This highly interesting species was obtained along with Synthecium campylo- 
carpum, from a depth of between 30 and 35 fathoms, off Port Jackson. 
Thecocladium, Allman. 
Thecocladium, Allman, Journ. Linn. Soc. Lond. (Zool.), vol. xix. p. 149, pi. xix. figs. 4, 5. 
Generic Character. Trophosome. — Branching stems set with disjunct hydro thecae 
and jointed at distant and uncertain intervals. Branches having their origin within 
the hydrothecse. 
Gonosome. — Gonangia ovate vesicles borne along the stems and branches and 
destitute of marsupium. 
The genus Thecocladium agrees with Thuiaria in the absence of internodes of 
definite length. It differs from it, however, in the very remarkable origin of the 
branches, which invariably spring from within the hydrothecse, passing out through the 
orifice, and then extending themselves externally. 
The proximal end of the branch in Thecocladium can be traced through the axis of 
a hydrotheca whose cavity it completely fills, while its coenosarc is continuous through 
the floor of the hydrotheca with the coenosarc of the stem or branch from which it 
springs. 
