GENUS ORBITOIDES. 301 



OrbitoHtes, so as to unite all the chamberlets of the same circle. If the " cast" represented 

 in Plate XXII, fig. 3, be really that of an Orhi hides, it appears to afford a confirmation of 

 this inference ; for we there see annular stolons* passing along between the successive rows 

 of sub-segments, on a different level from the radial peduncles, of which it will be seen that 

 usually only one shows itself at each extremity of the sub-segment. 



505. The shelly layer by which each of the lateral surfaces of the median disk is 

 covered-in, is nearly always thickest in the centre, thinning away more or less gradually 

 towards the margin, as shown in figs. 1, 7, 10, 1 1*. In some of those large, thick specimens 

 of 0. Fortisii, however, that occur in Scinde, in which the thickness of these superficial 

 layers is very great in proportion to that of the median layer, there is little or none 

 of such thinning-away towards the margin ; and occasionally we find the margin itself 

 to be covered-in by an extension of the outer portions of these superficial layers, which meet 

 each other in such a manner as completely to enclose the median layer and the inner portions 

 of the superficial layers, as is shown in fig. 15. This extension seems to mark the full 

 growth of the disk, and may be considered analogous to the closing-in of the spire of Niou- 

 mulina. — Each of these superficial layers is made up of a number of lamellae, having inter- 

 spaces between them, which are divided into flattened chamberlets by partitions formed by a 

 thickening of the lamellae, as shown in figs. 1, 2, 4, 6, 16. The form of these chamberlets 

 is very irregular, as shown in fig. 4, in the upper parts of figs. 8, 9, and in figs. 12, 13, 14 ; 

 it bears no relation whatever to the form of the chamberlets of the median disk ; and there is 

 no constant difference between the forms presented by 0. Mantelli and 0. Fortisii respectively. 

 These chamberlets are usually much flattened in the direction parallel to the median plane, 

 so that the thickness of many of them piled one on the other would not equal that of the 

 chamberlets of the median disk, as in shown in 0. Mantelli in fig. 6, and in 0. Fortisii in 

 fig. 16. But sometimes their height is considerably greater in proportion, and may equal, or 

 even surpass, that of the chamberlets of the median disk, as in the specimen of which a 

 vertical section is shown in fig. 11*. These chamberlets are usually ai'ranged in piles, one 

 upon the other, as is shown in figs. 2, 11, 16 ; and this arrangement seems to be peculiarly 

 definite when the superficial layers are traversed by the cones of non-tubular substance pre- 

 sently to be described. The chamberlets forming any one of these piles usually alternate in 

 level with those of the adjacent piles ; and each of them communicates by a pair of oblique 

 passages with two chamberlets, one lower and the other higher than itself, in each of the piles 

 surrounding its own, as shown in figs. 2, 6, 11. These communications are seen also, as brought 

 into view by a section parallel to the surface, in the upper part of fig. 9. The lamellae of 

 shell which form the floors and ceilings of these flattened chamberlets are themselves tubular, 

 as showm in figs. 2, 4, 6. Their tubuli are not so minute or so closely set as those of 

 Niimmtilina, OpermUna, or Ci/cloclypeus ; but they are much less coarse, on the other hand, 

 than those of Tinoporm, the arrangement of whose piled chambers is almost exactly similar 

 (t 394). This tubular structure (it seems desirable to mention) is often obscured, as in 



* I am by no means certain that these are not really the lepi-esentatives of annular canals 

 passing along interseptal spaces, rather than of stolon-passages uniting tlie cavities of the 

 chambers. 



