720 
THE VOYAGE OF H.M.S. CPIALLENGER. 
lion-segmented or only partially segmented spaces, forming tubular lacunae, the open ends 
of which are seen externally in the large, irregular, terminal orifices. The walls, except 
near the ends of the branching processes, are thick and laminated, and those of the outer 
faces of the chambers coarsely perforated, as in Gypsina. Owing to its parasitic habit, 
Polytrema is often associated with Sponges, and the terminal openings of the lacunae are 
not unfrequently armed with a. brush of siliceous spicula of various shapes, either entire 
or broken. 
The nature of the red colouring ! matter of the test of Polytrema does not appear to 
have been specially investigated ; but a few preliminary experiments suggest that it is an 
organic substance, and probably identical with that found by Merejkowsky in a large 
number of marine organisms, and named by him Zoonerythrine. 
The genus is best known by its typical species , Polytrema miniaceum, but Carter has 
described two other specific or subspecific modifications, namely, Polytrema cylindricum 
(Ann. and Mag, Nat. Hist., ser. 5, vol. v. p. 441, pi. xviii. fig. 1 , a.-g.), and Polytrema 
mesentericum {Ibid., p. 444, pi. xviii. fig. 3 , a.-h.): The former of these is stated to be 
“erect, cylindrical, consisting of a thick-round pillar developed from a slightly expanded 
base, dichotomously divided at the free end into two short thick branches, which are equal 
in length and opposite, terminating respectively in an expansion, from the centre of 
which radiate a number of more or less fragmentary sponge spicules ****** Cells 
at the ends of the branches bearing respectively a large aperture, similar in form, position, 
and margination to that of the cells of Planorbulina, which they further resemble in the 
form of their pore-tubulation,” &c, Polytrema mesentericum, the description of which 
is based upon a large “rolled specimen,” has a “massive test, composed of more or less 
erect, thick, meandering laminae united mesenterically.” 
The genus Polytrema is most abundant within the tropics, but extends as- far north 
as the Mediterranean and the Azores, and southwards to the southern shores of Australia. 
It is commonest in shallow water or at moderate depths, but is- occasionally met with as 
low down as 900 or 1000 fathoms. Little can be said with certainty as to its occurrence 
as a fossil. In the index of the Prodrome de Paleontologie, d’Orbigny enumerates 
upwards of fifty species of Polytrema, some of which appear under the genus Ceriopora 
in the body of the work, the whole of course being classed with Zoophytes. The 
geological range of these extends from the Devonian to the later Miocene or Pliocene 
period. Without a re-examination of the specimens, it is impossible to say how many of 
the species belong to the genus as at present constituted. In some of the Carboniferous 
Limestone shales of Scotland and the north of England there occurs a parasitic organism, 
Stacheia polytrematoides, 1 which closely resembles. Polytrema both as to external 
configuration and minute structure ; and it is by no means unlikely that further research 
may show that the relationship is even nearer than its name suggests. 
1 See — Monogr. Garb, and Perm. Foram., p. 118, pi. ix. figs. 9-13. 
