556 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



of the disk generally increases towards the periphery, the central portion of Cydo- 

 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 boundaries of the chambers are superseded by rounded 'punctations,' whilst the thick- 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 Cydoclypeus, 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 Orlltolites (^[ 20) as of rare occurrence ; in Cydoclypeus it is so common 

 that I have not yet met with specimens which are entirely free from it. 



97. Whilst agreeing with Orbitolites in those external features which result from 

 the cyclical mode of growth that is common to both forms, Cydoclypeus 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 have seen, 

 the general plan of structure of Orbitolites 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 Cydoclypeus 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 Cydodypeus-disk, 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. 



