FORAMINIFERA. 6l 



From a palaeontological point of view the only part of a 

 Foraminifer with which we have to deal is the shell or " test," 

 and there are several points to notice in this connection. 

 Firstly, as regards the actual composition of the shell, it is in 

 the majority of cases calcareous, or composed of carbonate of 

 lime, but it is rarely membranous, and it is not uncommonly 

 " arenaceous " that is, composed of particles of sand cemented 

 together by some animal substance. Secondly, the shells of the 

 Foraminifera may be divided into two natural groups, accord- 

 ing as their walls are, or are not, perforated by apertures or 

 " foramina" through which the pseudopodia are protruded. In 

 those calcareous shells in which the walls are not perforated 

 the substance of the test is " porcellanous," homogeneous, and 

 opaque-white when viewed by reflected light. In those^ calcar- 

 eous shells in which the walls are perforated by pseudopodial 

 foramina, the substance of the test is "vitreous," transparent, and 

 glassy. The arenaceous shells may or may not be perforated, 

 their texture in either case remaining the same. Thirdly, there 

 are some convenient, though arbitrary, distinctions to be drawn 

 from the form of the shell. The simplest form amongst the 

 Foraminifera, and that in which all alike primitively commence 

 their existence, is that of a single spheroid of sarcode capable 

 of secreting for itself a hard covering, as in Lagena (fig. 6, a) 

 and Orbulina (fig. 7). This simple state of things is rarely 

 retained throughout life, but the primitive 

 mass of sarcode usually repeats itself by bud- 

 ding, till a compound mass is produced, con- 

 sisting of a number of little spheres of sarcode, 

 surrounded by a common calcareous or are- 

 naceous envelope or test. Each bud of the 

 compound Foraminifer is surrounded by its p . 

 own shell, so that the whole comes to be com- wnversa. A simple 

 posed of a number of chambers, each contain- y^S^^- 

 ing a mass of sarcode. The partitions, how- Apennine beds) of 



., , S-rr Italy. D'Orbigny. 



ever, or "septa, between the different cham- 

 bers, are perforated by one or more apertures (fig. 6, d), through 

 which pass connecting bands of sarcode ; so that the sarcode 

 occupying the different chambers is united into a continuous 

 and organic whole. Each member of the colony may give out 

 its own pseudopodia through perforations in its investing wall 

 (fig. 6, <:), or the pseudopodia may be simply emitted from the 

 mouth of the shell by the last segment only (fig. 6, b). In any 

 case the direction in which the buds are thrown out by the 

 primordial spherule of sarcode is governed by a determinate 

 law, and differs in different species, the form ultimately assumed 



