GENUS TINOPORUS 225 



other localities have for the most part a spherical or spheroidal shape ; but a careful examina- 

 tion will generally make it apparent that this shape is derived (so to speak) from that last 

 mentioned, by the folding-inwards of its peripheral portion towards the centre of its lower 

 surface, so as to leave a deep cavity at that part of the sphere, — the relation of the two 

 forms being very much like that which exists between the expanded pileus of an Agaric, 

 and the same jji'/cus whilst still included within its volva. I cannot regard these diversities 

 of form as possessing any specific value ; since they depend entirely upon mode of growth, 

 and are not connected with any differences of internal structure. In whatever form the 

 T.vesicularis may present itself, it is recognised by the absence of projection or angularity, and 

 by a regular areolation over its whole surface, which a good deal resembles that of the cuticles 

 of many leaves, the areolae preserving a tolerably constant average of size, but being very inde- 

 finite as regards form. The septal divisions are marked by a very definite limbation, sometimes 

 formed of continuous ridges of shell-substance, and sometimes of rows of granules ; and in the 

 interspaces between these, under a sufficient magnifying power, minute punctations may be seen, 



393. Internal Structure. — When the structure of this organism is examined by means of 

 sections taken in different directions, it is found to be composed of an aggregation of minute 

 chambers of nearly uniform size, which are piled one upon another in pretty uniform layers, 

 eachof these presenting an approach to a concentric disposition (Plate XV,figs. 2, 3). Although 

 it is difficult to make out with certainty the arrangement of the first-formed chambers, yet it is 

 clear that as in other Foraminifera the point of departure is a spheroidal cell (fig. 3, a), wliich 

 soon comes to be surrounded by a cluster of secondary cells [b, c) derived from it by gemma- 

 tion. In what manner these are given off from the first, — whether by a spiral or by a cyclical 

 extension of the sarcode-body, — I have not been able to satisfy myself, on account of the 

 difficulty of precisely carrying the plane of section through this group of chambers. In T. 

 baculaliis I have been fortunate enough to do this in several instances, and have found that the 

 early growth is unquestionably spiral (fig. 12), — soon, however, giving place to the cyclical, as in 

 those varieties of Orhitolites whose growth commences after the same fashion (^ 180) ; and 

 whether this be or be not the case in T. vesicularis, it is indubitable that before long the 

 extension of the organism in diameter is effected by abudding-forth of new chambers from all 

 parts of the circumference, not with such regularit}', however, as to form distinct annuli like 

 those of Orhitolites. Whilst this extension is taking place peripherally, additional layers of 

 chambers are formed, as in Orbiculina, above and below the primordial cluster, meeting eacli 

 other on the equatorial plane ; and in this manner the increase of the organism in thickness is 

 effected. The grov>'th on the two sides of the equatorial plane, however, is seldom or never 

 symmetrical ; and that of the more convex portion seems continually tending to overpower 

 that of the opposite surface, so that the equatorial plane becomes more or less deeply concavo- 

 convex. I have reason to believe that this inequality is due to the attachment of the flat or 

 subconcave base to the surface of sea-weeds or zoophytes ; in virtue of wliich it will naturally 

 happen that the free side will grow faster than the other. It is by an excess of this predo- 

 minating growth that the spheroidal form is acquired, with its deep residual cavity, as just 

 now described. 



394. On more minutely examining tiie structure of the walls of the chambers and the 



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