722 
THE VOYAGE OF H.M.S. CHALLENGER. 
terminal orifices of the branches were crowded with loose sponge-spicules, which fell out 
on washing in a gentle stream of water. 
The distribution of Polytrema mmiaceum, var. alba, is probably coextensive with 
that of the typical red form, but numerically the specimens are comparatively scarce. 
Family X. NUMMULINID^E. 
The Family Nummulixida presents, as a whole, the highest phase of foraminiferal 
structure. It comprises a number of groups, characterised for the most part by possess- 
ing symmetrically-formed shells, usually of cliscoidal, lenticular, ovate, or fusiform 
contour, the chambers of which are arranged on a spiral plan, or occasionally in 
concentric zones. The shell-wall is in all cases finely tubulated, the pore-canals being 
as a rule of smaller diameter and more closely set than those observed in any other 
family of Foraminifera. In the more highly organised members of three Sub-families 
(or four if the Eozooxtxa be included) the test has a supplemental skeleton, variously 
developed, furnished with a canal system of greater or less complexity. 
The Sub-family Fusulixixze has no living representatives, but derives its chief 
interest and importance from its extraordinary abundance in Palaeozoic times. It embraces 
a series of perforate Foraminifera having precisely the same morphological range as 
the porcellanous type Alveolina, exhibiting every gradation of contour from subglobular 
or even lenticular to ovate and fusiform, the latter, as in Alveolina, being by far the 
most common and characteristic. The test has no supplemental skeleton and no canal 
system, and its minute structure appears to be about on the same level as Nonionina or 
Amphistegina, its bilateral symmetry and the form and position of the aperture suggest- 
ing an affinity to the former rather than the latter genus. 
The genus Nonionina displays the simplest structural features of the Polystomel- 
lixa, — a nautiloid shell with arched slit-shaped orifice. There is no supplemental 
skeleton, and almost the only salient peculiarity is the thickening of the shell-wall 
externally over the umbilical ends of the septal lines, so as to produce a sort of stellate 
sutural limbation ; and this is not by any means an invariable character. The varieties 
of Nonionina lead by insensible degrees to the genus Polystomella, the test of which 
becomes variously modified by the development to a greater or less extent of a supple- 
mental skeleton. In some of the Polystomellce the sutures are marked externally by 
minute orifices, which are the ends of the interseptal canals ; whilst in others the septal 
furrows are bridged over at intervals, and the canals open into the fossettes between the 
bridges. The aperture, which in the feebler varieties resembles that of Nonionina, 
becomes divided in more typical examples, and appears as an arched or Y-shaped 
row of pores. 
