GENUS TINOPOEUS: — INTEENAL STEUCTUEE. 
559 
213. External Characters. — The largest examples I have seen of Tino])orus Imvis 
present the form of a short truncated cone, much resembling the lower half of a sugar- 
loaf (Plate XXI. fig. 1, a), having its margins rounded otf, and attaining at the base a 
diameter of about one-tenth of an inch. The base commonly exhibits a slight central 
depression (fig. 1, h). Sometimes the cone is more depressed, and spreads out more 
widely at the base ; and in this case the basal concavity is usually wider and deeper. 
The examples I have seen from other localities have for the most part a spherical or 
spheroidal shape ; but a careful examination 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 Agaricus, and the same pileus 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. 
214. In whatever form the Tinoporus Icevis may present itself, it is recognized 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 areolse preserving 
a tolerably constant average of size, but being very indefinite as regards form. The 
divisions are marked by very definite boundary lines; and in the interspaces between 
these, under a sufficient magnifying power, minute punctations may be seen. 
215. 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, each of these presenting an approach to a concentric disposition (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 sphe- 
roidal cell (fig. 3, a), which soon comes to be surrounded by a cluster of secondary cells 
(5, c) derived from it by gemmation. 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. haculatus I have been fortunate enough to do 
this in several instances, and have found that the early growth is unquestionably spiral 
(fig. II), — soon, however, giving place to the cyclical, as in those varieties of Orbitolites 
whose growth commences after this fashion (^ 54) ; and whether this be or be not the 
case in T. Icevis, it is indubitable that before long the extension of the organism in 
diameter is effected by a budding-forth of new chambers from all parts of the circum- 
ference, not with such regularity, however, as to form distinct annuli as in Orbitolites. 
Whilst this extension is taking place peripherally, however, additional layers of chambers 
are formed, as in Orbiculina, above and below the central nucleus, meeting each other 
