22 THE VOYAGE OF H.M.S. CHALLENGER. 



substance in these disks is far greater in proportion to the cavitary system, than in 

 OrhitoUtes tenuissima ; at least half the breadth of each annulus being occupied by the 

 inter-annular septum, and the partitions that sejsarate the adjoining chamberlets being 

 also much thicker. These partitions are best brought into view by a concentric fracture 

 separating one annulus from another, so that the outer series of chamberlets is laid open 

 on its central aspect, as shown in fig. 5 ; but such a separation is much less easy in this 

 type than in the preceding, in consequence of the much larger surface of adhesion 

 between the successive annuli. When the cavitary system is laid open by a section in 

 the radial direction, so as to traverse a succession of annuli (fig. 6), there is seen in each 

 of the partitions that divide the chamberlets a large fissure on its peripheral side, by 

 which the adjoining chamberlets of the same annulus are brought into connection. 

 This fracture also shows that the columnar chamberlets of the marginal portion of the 

 disk are not straight, but arcuate ; their two extremities bending inwards, or towards its 

 centre. 



It is only, however, by reducing the thickness of the disk by grinding, so that it can 

 be examined by transmitted light, that its internal structure can be properly traced out. 

 If only one of its surfaces be ground away, so that the sectional plane passes near the 

 other, it will traverse the chamberlets, but not the passage-system which connects them, 

 as is seen in PI. IV. figs. 1, 2, 3 ; but when this plane is made, by grinding from both 

 surfaces alike, to pass through the middle of the thickness of the disk, the communi- 

 cations between the chamberlets are brought into view, as shown in fig. 4. Here 

 we see the flask-shaped primordial chamber a, opening at its neck into the circumambient 

 chamber b, which almost completely surrounds it ; while from the other end of this, 

 there issues a passage that leads into the undivided chamber c. 



Taking this chamber as our starting-point for comparison with the "orbiculine" por- 

 tion of the disk of OrhitoUtes tenuissima, we find the parallelism extremely close. The 

 septal plane which bounds it externally is traversed by two passages that lead into two 

 chamberlets d, which are connected with each other laterally by a passage left in the 

 partition between them. The septal plane that closes-in these two chamberlets is tra- 

 versed by five radial passages, leading to as many chamberlets in the next row e ; of these 

 passages two proceed from each of the chamberlets in row d, and one from the passage 

 that connects them ; and all five chamberlets are brought into lateral connection with 

 each other by passages left in the radial partitions, as shown in PL III. fig. 6. The 

 next septal plane is traversed by a radial passage from each of the passages of 

 communication between the chamberlets of the preceding series, and also by passages 

 from the chamberlets themselves ; and as each of these leads to a chamberlet of the 

 succeeding row, the number of these is further increased. The same mode of growth 

 continues, until the lateral extension of the rows of chamberlets (each representing a 

 single "peneropline" chamber) brings together their extremities so as to complete the circle; 



