EEPOET ON THE EOE AMINIFEE A . 
723 
Turning now to the Nummulitijsee, — the little Carboniferous fossil, Archcediscus, 
exemplifies the lowest type of Nunrmuline structure, and stands in very much the same 
relation to the genus Nummulites that Spirillina bears to the higher Rotalines. The 
shell is lenticular, and consists of a non-segmented tube of gradually increasing diameter, 
coiled upon itself somewhat unsymmetrically. The wall of the tube is extended laterally 
over the two faces of the test in the same way as the alar prolongations of the chambers of 
the true Nu mm ulite. There is no secondary skeleton, but the walls are thick and laminated 
and finely tubulated. The genus A rnph istegina presents a structure considerably in 
advance of Archcediscus, corresponding with that of Pulvinulina in the Rotaline group. 
The test is lenticular, the two faces being unequally convex ; the segments are narrow 
and equitant, but their alar extensions on the inferior side are each divided into two 
portions by a deep constriction, so that the umbilical ends form a series of distinct lobes. 
The aperture resembles that of the Rotcdince, — an arched fissure at the inner margin of the 
final segment on the inferior side. The typical aspect of the genus Opercidinct is that of 
a thin, complanate, planospiral shell of somewhat large dimensions, the convolutions of 
which are all visible externally, though the earlier ones are more or less embracing ; the 
chambers are usually very numerous, narrow, and undivided, and the aperture a simple 
cleft at the inner margin of the terminal segment. Heterostegina displays similar general 
features, but the chambers are subdivided by transverse septa, and the aperture takes the 
form of a row of pores on the exposed septal face. The genus Nummulites is closely 
related to Opercidina, but exhibits a further advance in structure and organisation. The 
true Nummulite has a cliscoidal test, the two faces of which are as a rule equally convex, 
formed of several convolutions, each of which completely invests its predecessor. The 
spire does not increase in diameter so rapidly or so regularly as that of Opercidina, and 
in the larger varieties the final convolution becomes gradually contracted at its peripheral 
margin, until it closes in the shell. The septa are double, and are traversed by a system 
of canals, which communicates with that of the marginal portion of the supplemental 
skeleton. 
The two genera constituting the Sub-family Cy cloclypeinze afford instances of an 
annular instead of a spiral mode of growth. The test both of Cycloclypeus and Orbitoides 
is discoidal and bilaterally symmetrical, and either lenticular in contour or complanate 
and thickened only at the centre. It consists primarily of a median layer composed of 
chamberlets arranged in concentric zones. In Cycloclypeus this median plane of chambers 
is thickened on both sides, chiefly near the middle, by layers of finely tubulated shell 
substance ; whilst the test of Orbitoides presents similar lateral masses, composed of 
layers of minute chamberlets irregularly combined. In either case the canal system is 
traceable both through the central and superficial portions of the shell. 
Concerning Eozoon and the provisionally constituted Sub-family Eozoonin.e, it is need- 
less to speak, so long as the claim of the former to be ranked as a member of the animal 
