50 



OF THE FORAMINIl'ERA GENERALLY, 



This is the case tiiroughout the 'porcellanom series and with the lower forms of the vitreous 

 series. But in the highest types of the latter we find that each chamber of the spire has a 



Fig. V 



Fig. VI 



Fig. VIII. 



complete shelly envelope of its own, the new segment forming a posterior wall, which 

 applies itself to the anterior wall of the preceding segment, so that each septal plane 

 (except that of the last chamber) is composed of two lamella;, as is shown in Fig. VIII. 

 — The same difference exists among the Polt/tlialauiia formed upon the cijdkal plan of growth, 

 which is ordinarily the result of the gemmation of the primordial segment not on one side 

 only, but from every part of its circumference, and the subsequent repetition of the same 

 mode of increase (see Fig. XXIV, p. 108). For in the cyclical forms of the porcelhnio/'s series 

 the successive annuli are joined, each to its predecessor, in such a manner that the external 

 wall of the latter serves as the internal wall of the former, as is seen along the lines a a, a a, 

 Fip-. V; and the septa dividing the adjacent chamberlets of the same annuli are also single. 

 In the cyclical forms of the vifreo//s series, on the other hand, each chambcrlet has its own 

 proper wall, as is shown in Fig. VI : so that not merely the annular but the radial septa are 

 all double. 



63. But this is by no means all ; for in the higher types of the hyaline or vitreous series 

 we frequently meet with an " intermediate" or " supplemental" skeleton, formed by a secondary 

 or exogenous deposit upon the outer walls of the chambers, by which they receive a great 

 accession of strength. This deposit not only fills up what w^ould otherwise be superficial 

 hollows at the junctions of the chambers, or (as in PolydomeUa) at the umbilical depression, 

 but often forms a layer of considerable thickness over the whole surface, thus separating 

 each whorl from that which encloses it (Fig. YIII, d) ; and it is .sometimes prolonged into 

 outo-rowths that give a very peculiar variety to the ordinary contour, as in some varieties 

 of Botalia and FohjstomeUa, but most characteristically in Calcanna (Plate XIV, figs. 1, 2, 8) 

 and the stellate form of Tinopori's (Plate XV, figs. 5—9). This intermediate or supplemental 

 skeleton, wherever developed to any considerable extent, is traversed by a set of "canals," 

 which are usually arranged upon a systematic plan, and are sometimes distributed with 

 considerable minuteness. The passages which make up this " system" are not true vessels, but 

 are mere sinuses, left in some cases by the incomplete adhesion of the two contiguous walls 

 which separate the adjacent chambers, and in other cases apparently originating in the 

 incomplete calcification of the sarcode which forms the basis of the solid skeleton ; certain 



