GENUS CYCLOCLYPEUS. 293 



elevated), as bj' the substitution, along the septal lines, of the hyaline non-tubular sliell- 

 substancc for that which is rendered opaque by its tubularity '{% 58). And further, the 

 forms of the two genera ordinarily differ in this — that whilst the centre of Orbitulifes is 

 usually rather depressed than elevated, and the thickness of the disk generally increases 

 towards the periphery, the central portion of Cyclodypem always presents a knobby elevation, 

 on the surface of which the oblong boundaries of the chambers are superseded by rounded 

 spots of non-tubular shell-substance (sometimes rising into tubercles), whilst the thickness of 

 its disk gradually diminishes towards its margin, where it is so I'educed as to come to a sharp 

 edge. In older specimens of Cydodiipeus, the boundary markings of the chambers are scarcely 

 distinguishable, save near the margin ; their concentric annuli are marked out, however, 

 by rows of spots resembling those of the central eminence, which will be presently shown 

 (*]y 497) to be the bases of cones of non-tubular substance (fig. 5, d, d, d). 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 OrbifoUtes (^ 171) as of rare occurrence; in Cijdodypeii>i it is 

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



495. Internal Structure. — Whilst agreeing with Orbitolites in those external features 

 which result from the cydical mode of growth that is common to both forms, Cydodypeus 

 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. 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 are covered-in above and 

 beneath by compact plates of shell, which are thicker towards the centre, thinner towards the 

 circumference (Plate XIX, fig. 2). The typical form of these chambers seems to be a parallelo- 

 gram with its angles rounded off, whose sides are to each other as I5 to 1, or as 2 or even 3 to I, 

 the longest side lying in the direction of the radius of the disk ; but owing to the variation 

 in the leiiyth of the chambers which results from the before-mentioned irregularity in the 

 bi'eadth of the annuli, the breadth of the chambers remaining more constant, their pro- 

 portions vary greatly in diflerent parts of the same annulus, or in adjacent parts of different 

 annuli, as shown in fig. 7. I have occasionally met with chambers whose length was to their 

 breadth as 4 to 1 . The vertical thickness or height 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. But although the existence of only a single layer of chambers is obviously the 

 rule in this type, yet exceptions to it are not unfrequent ; a subdivision of the entire stratum 

 into two or even three presenting itself when its thickness is above the average ; whilst 

 occasionally one or two chambers only are thus subdivided. 



496. The cavity of each chamber is surrounded by a proper wall of its own, quite 

 distinct from that of the chambers which it adjoins ; and hence the septum by which each 

 chamber is divided from the adjacent one on cither side, is formed of at least two lamella;. 

 These come into close contact with each other at the junction of the vertical septum with the 



