402 



Messrs. Carpenter and Brady on 



[Apr. 22, 



inch. The character of their external surface differs considerably in dif- 

 ferent individuals ; but the Author gives reason for believing that it was 

 originally tuberculated, like a mulberry, and that the departures from this 

 have been the result of subsequent abrasion. The entire sphere is com- 

 posed of a great number of concentric layers, all of which, except the inner- 

 most, are arranged with very considerable regularity around a central "nu- 

 cleus," which consists of five chambers, disposed in rectilineal sequence, 

 thus unmistakeably indicating the Foraminiferal character of the organism, 

 which might otherwise have remained in doubt, on account of the entire 

 divergence from any known type presented in the structure of the concen- 

 tric layers. The first of these layers is moulded (as it were) on the exterior 

 of the nucleus, and partakes of its elongated form ; but the parts of every 

 additional exogenous layer are so arranged as to bring about a gradual 

 approximation to the spherical form, which is afterwards maintained with 

 great constancy. Each layer may be described as consisting of a lamella 

 of " labyrinthic structure " (that is, of an assemblage of minute cham- 

 berlets, whose cavities communicate freely with one another), separated 

 from the contiguous lamellae by an " interspace," which is traversed by 

 " radial tubes," that pass from each lamella to the one external to it. All 

 these structures, in common with the chamber-walls and septa of the 

 "nucleus," are built up by the aggregation of sand- grains of very uniform 

 size. These sand-grains are found to consist of Phosphate of lime; and 

 they seem to be united by a cement composed of Carbonate of limey 

 which was probably exuded by the animal itself. Although there is a 

 very general uniformity in the thickness of the successive layers, the pro- 

 portion of their several components varies considerably in different parts 

 of the sphere. In those which immediately surround the nucleus, the 

 solid lamellae, which are composed of labyrinthic structure, are compara- 

 tively thin; whilst the interspaces which separate them from one another 

 are very broad, so that the radial tubes which traverse these interspaces 

 are very conspicuous. As we pass outwards, we find the labyrinthic lamellae 

 increasing in thickness, whilst the breadth of the interspaces diminishes in 

 the same degree, until we meet with layers in which the interspaces are 

 almost entirely replaced by labyrinthic structure. With this increased 

 development of the labyrinthic structure in the concentric lamellae them- 

 selves, we find it extending between one lamella and another, as an invest- 

 ment to the radial tabes; thus forming "radial processes" of a sub- 

 conical form, which occupy a considerable part of what would otherwise 

 be the interspaces between the successive lamellae. Still every lamella 

 is separated from that which invests it (except where brought into con- 

 nexion with it by its radial processes) by a system of cavities, which are 

 in free communication with each other, and which may be collectively de- 

 signated the "interspace-system ;" and from this system the labyrinthic 

 structure of the investing lamella is entirely cut off by an impervious wall, 

 which bounds it upon its inner side ; whilst its chamberlets open freely upon 



