556 
DR. carpenter’s RESEARCHES ON THE FORAMINIFERA. 
of the disk generally increases towards the periphery, the central portion of Cyclo- 
clypeus always presents a knobby elevation, on the surface of which the oblong 
boundaries of the chambers are superseded by rounded ‘punctations,’ whilst the thick- 
ness of its disk gradually diminishes towards its margin, where it is so reduced as to 
come to a sharp edge. In older specimens of Cycloclypeus, the boundary-markings 
of the chambers are. scarcely distinguishable, save near the margin ; their concentric 
annuli are marked-out, however, by rows of ‘ punctations,’ similar in appearance to 
those of the central eminence. In either case, it is usually observable that the 
breadth of the annuli is far from constant; and that the annuli are not unfrequently 
incomplete, extending round only a portion of the disk. This irregularity has been 
noticed in Orhitolites 20) as of rare occurrence; in Cycloclypeus it is so common 
that I have not yet met with specimens which are entirely free from it. 
97 . Whilst agreeing with Orhitolites in those external features which result from 
the cyclical mode of growth that is common to both forms, Cycloclypeus presents as 
wide a contrast to it in every other feature of its organization, as is anywhere known 
to exist within the limits of the Foraminiferous group. For whilst, as we liave seen, 
the general plan of structure of Orhitolites removes it but little from the grade of 
Sponges — the several segments of its aggregate body being but very imperfectly 
separated one from the other, and the shell which grows-up in the midst of them 
having no discoverable organization, — that of Cycloclypeus closely approximates to 
the Nummulitic type, in which the successive segments are as completely isolated as 
they can be without entire disconnection, and in which, by the peculiarly-elaborate 
construction of the shelly covering, a special provision is made for their independent 
nutrition. 
98. On making horizontal and vertical sections of the Cycloclypeus-iW&k, its central 
plane is found to be occupied by chambers, disposed (ordinarily in a single layer) in 
concentric annuli ; these being covered-in above and beneath by compact plates of 
shell, which are thicker towards the centre, thinner towards the circumference 
(Plate XXX. fig. 1). The typical form of these chambers seems to be a parallelo- 
gram with its angles rounded off, whose sides are to each other as 1^ to 1, or as 2 or 
even 3 to 1, the longest side lying in the direction of the radius of the disk ; but 
owing to the variation in the length of the chambers which results from the before- 
mentioned irregularity in the breadth of the annuli (^ 96), the breadth of the cham- 
bers remaining more constant, their proportions vary greatly in different parts of the 
same annulus, or in adjacent parts of different annuli, as shown in Plate XXIX. 
fig. 12. I have occasionally met with chambers whose length was to their breadth 
as 4 to 1 (Plate XXXI. fig. 3). The vertical thickness or depth of the chambers, 
seems usually to be pretty constant in different parts of the disk, except near its centre; 
the thinning-away towards its margin being due, not so much to a diminution in 
the vertical height of the chambers, as to the reduction of the thickness of the shelly 
plates that enclose them above and below. 
