IV 
THE VOYAGE OF H.M.S. CHALLENGER. 
tively few examples had hitherto been known, are those to which belong the genera 
Cryptolaria and Grammaria, as well as a new and very interesting genus to which I 
have assigned the name of Perisiphonia, and of which the collection contains two 
species. 
An examination of the specimens by which these genera are represented has shown 
that they possess a remarkable and hitherto unsuspected type of structure, rendering 
necessary the institution for them of two new families, while to one of these families 
must also be referred certain other genera with which zoologists have long been familiar, 
but in which the essential character of the family had been overlooked. 
Idia, hitherto known only by the very deficient description and figure of Lamouroux , 1 
and of which only a single species, Idia pristis, has as yet been discovered, is repre- 
sented by examples which show that it is constructed on a type quite unique among 
the Hydroida, and one which demands the allocation of it to a special Hy droid 
section. To this section I have assigned the name of Thalamophora. 
Among other families largely represented is that of the Haleciidse, with not only many 
new species, but with one form which must be referred to a new generic type, and which 
is rendered especially interesting by the fact that the colony is provided with bodies 
which admit of a close comparison with the sarcostyles and sarcothecse of the Plumu- 
larinse. Similar bodies are also borne by the two species of Perisiphonia already referred 
to, and by a species referable to the new genus Hypo'pyxis. The presence of these 
bodies, formerly supposed to be confined to the Plumularinse, with a very few forms 
belonging to other groups, has thus been shown to be by no means so limited as had been 
imagined. 
In two species, one of which is referable to Sertularia and one to Thuiaria, the 
specimens afford abundant evidence of the fact that the hydranths are incapable of 
complete retraction within the hydrothecse. In both of these the body of the hydranth 
is connected with the wall of the hydro theca by ectodermal bands quite similar to those 
which in most Hydroid trophos'omes connect the coenosarc with the walls of the perisarcal 
tube, or the blastostyle with those of the gonangium. In at least one of these the 
tentacles of the hydranth are each provided at its base with a prominent cushion loaded 
with thread-cells, and forming a defensive battery in which we can scarcely avoid seeing 
a provision, the object of which is to act as a compensation for the comparatively 
unprotected condition of the hydranth. 
The curious genus Synthecium, in which the gonangia spring from within the cavity 
of the hydrotheca, is represented by two new species, both from the Australian seas, thus 
extending our knowledge of the range of this genus, which had been previously known 
only through specimens obtained from the region of New Zealand. There also occur fine 
1 Mr. Hincks has since sent me a specimen of this Hydroid from the Mergui Archipelago, while I am also indebted 
to my late lamented friend Mr. Busk for an opportunity of examining singularly fine specimens from the Persian Gulf. 
