200 
DR. carpenter’s RESEARCHES ON THE FORAMINIFERA. 
be formed, the prolongations of sarcode issuing from the several pores of the pre- 
ceding zone coalesce, so as to form a complete zone of segments and connecting 
stolons around the margin of the previously-foi’med disk ; and that the deposit of 
calcareous matter forming the shelly walls of the cells and passages, takes place upon, 
or rather in, the superficial portion of this zone of sarcode. But I cannot find any 
evidence in the ordinary growth of the disk, that the sarcode extends itself over the 
surface of the portion previously formed ; although occasional appearances will be 
hereafter described 53), that seem to indicate that it may do so. 
21. It is a fact of much importance in the due appreciation of the relations of 
OrbitoUte and its allied forms to other tribes of Foraminifera, that the calcareous 
partition which separates each cell of any one zone from its neighbours on either 
side, is not double, but single. And this is in great part the case, even with regard 
to the partitions that separate the cells of successive zones ; the inner or central 
boundary of one being chiefly foi-med by the peripheral wall of the other. It is not 
easy even in thin sections to distinguish the boundary between the walls of one zone 
and those of another, so absolutely continuous do they appear to be. But it not 
unfrequently happens, that in fracturing these disks, their component zones come 
apart from each other, along their natural lines of junction, so as to disclose the real 
inner (or central) margin of the outer segiuent, which then presents a set of wide 
apertures, through which we look at once into its cells; thus proving their incom- 
plete enclosure by proper walls on that side (Plate V. fig. \,jf). Thus in the forma- 
tion of each new zone, the calcareous envelope seems to be only generated where the 
sarcode is not already in contact with a solid wall. 
22. There cannot be any reasonable doubt, that the number of concentric zones 
which any disk may present, is entirely determined by its stage of growth, and that 
it affords no basis whatever for specific distinction. Just as in the case of the con- 
centric layers of wood in the stem of a tree, a minute nucleus, surrounded by only a 
single annulus of cells, may come in time to be the centre of a large disk consisting 
of many scores of concentric zones. Although, as already stated (^ 17), most of the 
Orbitolites fortned upon this simple type are of comparatively small size, yet there 
does not seem to be any definite liniit to the multiplication of zones; for I possess 
specimens attaining T5 of an inch in diameter, and consisting of about forty zones 
(much larger, therefore, than the younger zones of the complex type), in which there 
is no appearance of any departure from the original mode of growth. That com- 
paratively few specimens, however, attain so large a size upon this simple type of 
structure, is due, I believe, to the circumstance that they early tend to develope 
themselves upon the more complex plan which I shall presently describe. 
23. Although I have spoken of these disks as typically plane or nearly so (there 
being usually no great difference between the thickness of their central and that of 
their perij)heral parts), yet it not unfrequently happens that the successive zones gra- 
dually increase in thickness from within outwards (as is shown in Plate V. fig. 5), 
