GENUS TINOPOBUS: INTERNAL STEUCTUEE. 559 



213. External Characters. The largest examples I have seen of Tinoporus Icems 

 present the form of a short truncated cone, much resembling the lower half of a sugar- 

 loaf (Plate XXI. fig. 1, #), having its margins rounded off, and attaining at the base a 

 diameter of about one-tenth of an inch. The base commonly exhibits a slight central 

 depression (fig. 1, V). 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. baculatus I have been fortunate enough to do 

 this in several instances, and have found that the early growth is unquestionably spiral 

 (fig. 11), soon, however, giving place to the cyclical, as in those varieties of OrUtolites 

 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 OrMtolites. 

 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 



