GENUS ORBITOLITES : VARIATIONS IN MODE OF GROWTH ; MONSTROSITIES. 221 



a considerable size on this duplex type ; and its edge then presents two rows of 

 rounded prominences with pores between them, those of the upper and lower rows 

 alternating with each other, as is well seen in the vertical plate of the monstrosity 

 represented in Plate IX. fig. 10. It is on a disk of this type that Professor EHREN- 

 BERG has founded his genus Amphisorus, which I cannot regard as even specifically 

 distinct from the ordinary Orbitolite. 



60. The next variety to be noticed, consists in a complete absence of regularity in 

 the disposition of the columnar cells of the intermediate stratum, so that they present 

 an assemblage of indefinitely-shaped passages, communicating with each other in 

 various directions. This variety is chiefly interesting, as showing how little import- 

 ance is to be attached to smaller deviations of the same kind. The most remarkable 

 example of it which I have met-with, is represented in Plate V. fig. 7, c d. In Plate V. 

 figs. 1 1, 12 are represented two examples of irregularity in the disposition of the super- 

 ficial and intermediate cells in the zones immediately surrounding the centre. 



6 1 . Lastly, I have to mention, that the septa dividing the contiguous cells of the same 

 zone are occasionally deficient, so that the interior of the zone is a continuous circular 

 passage, with only slight indications of the normal divisions. In such a case, it is ob- 

 vious that the ring of sarcode must have been everywhere of nearly uniform thickness, 

 showing no division either into horizontal or into vertical segments ; and it may not 

 be thought improbable that this is its first condition in every case, and that its seg- 

 mental division is a subsequent process, so that the shelly investment, if formed pre- 

 viously to the segmentation, will have the character of incompleteness just described. 

 I cannot help suspecting, that the peculiar groove around the margin of the Feejee 

 specimens formerly noticed (^[25.), is referable to a still greater incompleteness of 

 the production of the calcareous investment around the newly-forming zone. 



62. Monstrosities. Besides those departures from the normal type of growth, which 

 have been described as variations or irregularities, there are certain others of rarer 

 occurrence, which can only be regarded as ' monstrosities by excess ;' consisting in the 

 production of one or more incomplete secondary disks by outgrowth from the first. 

 In the specimen represented in Plate IX. fig. 8, the secondary disk forms a half-circle 

 BD, of about the same diameter with the primary AC, and is superposed vertically upon 

 the latter, the plane of junction passing through its centre. In the 



specimens represented in figs. 7 and 9, the secondary disk is relatively j 



smaller, extending only from the centre to the margin of the primary, B 



but still meeting it nearly at right angles. In the specimen of which two different 

 aspects are shown in figs. 5, 6, it would seem difficult to say which is the primary and 

 which the secondary disk, and it would be more correct to describe 

 the entire structure as consisting of a single half-disk AB and of A_ 

 two half-disks BC and BD, meeting each other at an acute angle 

 CBD, neither of them being in the same plane with the single half-disk, but both of 

 them meeting it at similarly obtuse angles ABC and ABD. The opening of one of the 

 MDCCCLVI. 2 G 



