110 FAMILY MILIOLIDA. 



the chamberlets 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 distin- 

 guish 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 fracturw(i these 

 disks, their component zones come apart from each other along their natural lines of junc- 

 tion, so as to disclose the real inner margin of the outer zone (Plate IX, fig. 1, g, (/), which is 

 then found to present a set of wide fissures, through which we look at once into its cham- 

 berlets, thus proving their incomplete enclosure by proper walls on that side. 



1 60. 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 aftbrds no 

 basis whatever for specific distinction. Just as in the case of the concentric layers of wood 

 in the stem of an exogenous tree, a minute " primitive disk," surrounded only by a single 

 annulus of chamberlets, may come in time to be the centre of a large disk consisting of 

 many scores of concentric zones. Although, as already stated (^ 156), most of the Orbi- 

 tolites 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 '25 

 of an inch in diameter, and consisting of about fifty zones (much larger, therefore, than the 

 younger disks of the complex type), in which there is no appearance of any departure from 

 the original mode of growth. 



161. Structure of the Animal.— The; entire animal body, obtained by the decalcification of 

 the shells of specimens that have been taken alive and preserved in spirit (Plate IV, fig. 14), 

 is composed, of a numerous assemblage of minute segments," arranged at tolerably regular 

 intervals in concentric zones, around what it may be convenient to term the " primitive 

 mass." This primitive mass consists of a pear-shaped " primordial segment" (Plate IV, 

 figs. 1 6 — 20, a) occupying the central or primordial chamber, from the small extremity of 

 which a peduncular process [d) extends, that dilates again into the still larger segment h, b, 

 which, from its almost completely surrounding the former, may be conveniently designated 

 the " circumambient segment." Thus, the " primitive mass" very closely resembles what the 

 body of a young Miliola would be, if the segment first budded-oft" from the primordial 

 segment were prolonged, so as completely to surround it (1 49, Plate IV, fig. 4, b). From 

 the outer margin of this circumambient segment there radiate a number of slender pedicles 

 (c), which presently enlarge into as many columnar segments, having a circular or oval base, 

 and these are united to each other laterally by a connecting band or " stolon" {h, h), which 

 passes entirely round the " circumambient segment" and forms a complete annulus. It is 

 usually from the portions of this "stolon" which intervene between the segments of each 

 annulus of sarcode, that the radiating pedicles (fig. 23, e, c, e) are given off, which go to 

 originate the next zone ; but sometimes a pedicle is given oft' from one of the segments (as 

 shown at e',e'), which goes to form a segment (c", c") in the next zone, that is interpolated 

 between two which have originated from the intervening annular stolon ; and it is in this 

 manner that the numJjer of segments in successive zones receives an increase, the size of those 

 segments not undergoing any considerable augmentation. Each successive zone exhibits 

 precisely the same structure ; and it is obvious that the radiating pedicles of the outermost 



