396 MR. F. CHAPMAN ON THE 
PoLYTREMA PLANUM, Carter. (PI. 35. figs. 2 & 4.) 
Polytrema planum, Carter, 1876, Ann. & Mag. Nat. Hist. ser. 4, 
vol. xvii. pp. 211, 212, pl. xiii. figs. 18, 19. 
Gypsina melobesoides, Carter, 1877, Ann. & Mag. Nat. Hist. ser. 4, 
vol. xx. p. 172. 
Polytrema miniaceum, var. involva, Chapman, 1900, Journ. Linn. Soe., 
Zool. vol. xxviii. pp. 17, 18, pl. 2. fig. 5, and text-figure 2. 
P. planum, Carter, Chapman, 1901, Ann. & Mag. Nat. Hist. ser. 7, 
vol. vii. pp. 82, 83. 
P. planum, Chapman, 1901, Journ. Linn. Soc., Zool. vol. xxviii. 
pp- 201, 202, pl. 20. figs. 6, 7. 
This freely-growing foraminifer produces extraordinarily large 
masses of calcareous rock at Funafuti. At first sight they were 
thought to be calcareous alge, for their habit of growth is very 
similar to encrusting forms of Lithothamnion and Lithophyllum. 
A microscope-section of the organism, however, at once reveals 
its relationship with Polytrema and Gypsina. It is extremely 
difficult to decide from an examiuation of the external surface 
whether the specimens are foraminifera or alge, unless one has 
a special knowledge of the minute differences met with in the 
respective groups. 
It is not uncommon to find this organism forming an encrusting 
mass of about 5 centimetres square. One example from Tutanga 
(60-100 fathoms) measures about 7 x5 centimetres. Another 
piece of calcareous rock, composed almost entirely of laminar 
growths of P. planwm, has a measurement of 13 xX5 x4 centi- 
metres. 
As regards the depths at which this peculiarly interesting 
foraminifer is found, the limits of its best development are from 
about 80 to 200 fathoms. 
