FROM THE FUNAFUTI ATOLL. 17 
is the result. Should it become cramped in its surroundings, the 
rhizopod makes a thick base with short and strong prolongations. 
When the Polytrema affixes itself to a detached fragment affected 
by currents which roll it over, the organism forms a thin en- 
crustation, which in many cases is in turn encrusted by a Litho- 
éhamnion, then again by Polytrema, and so on, until the nodules 
often measure as much as 5 centimetres in diameter. This latter 
mode of growth in Polyérema has hitherto been unnoticed, and 
seems to merit a distinct varietal name*. An intermediate 
stage between this encrusting Polytrema and the arborescent 
typical form is represented by Carter’s P. mesentericum +. 
The red colour of Polytrema is apparently very little, if at all, 
dependent upon the influence of light. Some of the finest 
coloured specimens occur at considerable depths, and it is 
remarkable that off Funamanu Id. (Beacon Island), at a depth 
of 150 fathoms, Polytremata of the deepest and richest colour 
were found. 
Although commonly found in shallower water, P. miniaceum 
has been recorded in the ‘ Challenger’ soundings from a depth as 
great as 1000 fathoms. Egger found it ranging down to a depth 
of 411 metres (224 fathoms) near Mauritius. Specimens recorded 
from the Caribbean Sea by Goés were found at 115 fathoms. 
PoLyTREMA MINIACEUM (Pallas), var. INVOLVA, nov. (Pl. 2. 
fig. 3, & text-fig. 2.) 
Test consisting of a rudely parallel series of acervuline chamber- 
lets disposed round a nucleus, or partly encrusting an adventitious 
particle. The organism in this stage shows little tendency to 
send forth prolongations, but seems content with forming mere 
rounded prominences or papille at intervals on the surface of 
growth. ‘This variety often shows a remarkable tendency to 
alternate in its growth with layers of Zithothamnion, thus 
building up nodular masses of a composite nature {. The 
nodules sometimes reach the dimension of 5 centimetres. 
As previously noticed, the form named Polytrema mesentericum, 
Carter, may be in part comparable with this form. It is 
probable, however, that this was the basal portion of a large 
* See p. 1. 
t+ Ann. Mag. Nat. Hist. ser. 5, vol. v. (1880) p. 444, pl. xviii. figs. 3 a-h. 
¢ Another species has been noticed at Funafuti which occurs in the lime- 
stones alternating in its growth with Lithothamnion, namely Gypsina inherens 
(Schultze); but it does not form anything like so complete an, investment as 
P. miniaceum var. involva. 
LINN. JOURN.—ZOOLOGY, VOL, XXVIII. 2 
