FOSSIL FOEAMINrFERA. 



187 



violet, blue, &c. Its consistence is variable ; it is composed of minute globules, the aggregation 

 of which determines the general tint. It is sometimes entire, round, and without segments, as 

 in Gromia, Orhulina, &c., which represent, at all ages, the embryonic state of all the other genera. 

 They increase, without doubt, by the entire circumference. When the body is divided by lobes 

 or segments, the primary lobe, as in the permanent condition of the Gromia, is at first round or 

 oval, according to the genus ; once formed it never enlarges, but is enveloped externally by 

 testaceous matter ; it may be compared to a ball on which is applied a second larger one, then a 

 third still larger, and so on during the life of the animal. 



The annexed figure of the animal oi Nummulina (as given by MM. Joly and Leymerie) will 

 serve to convey a general idea of the living Foraminifera. 



THE ANIMAL OF THE NUMMULINA. 



The segments, as the body increases, are agglomerated in six different ways, and these modi- 

 fications are the basis of M. D'Orbigny's classification. The discoidal forms, as the Rotalia, 

 Rosalina, Cristellaria, &c. are involuted like the nautilus, and divided by septa or partitions, the 

 diflferent lobes of the body occupying contemporaneously every chamber, and being connected by 

 a tube or canal that extends through the entire series. In the spiral forms, the Textilaria, &c. 

 the same structure is apparent. These two groups are the most abundant in the cretaceous 

 strata ; many beds of the white chalk consist ahnost wholly of the aggregated shells of the 

 RosalinsB, Kotalis, and TextilariEe.' Whatever the form of the body, the filaments always consist of 

 a colourless matter as transparent as glass ; they elongate from the base to six times the diameter 

 of the shell. They often divide and subdivide, so as to appear branched. Though alike in form 

 in the different genera, they vary much in theu- position. In some they form a bundle which 



' See Wonders of Geology, p. 299. 



