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-formed 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 

 Orbitolite 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 formed 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 segment, 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. I,//). 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 stern 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 formed upon this simple type are of comparatively small size, yet there 

 does not seem to be any definite limit to the multiplication of zones ; for I possess 

 specimens attaining -15 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 peripheral 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), 



