132 PROTOZOA. 



by Dr H. B. Brady, is a much more restricted one. The test is 

 always calcareous and perforate, and consists of a few inflated 

 chambers arranged spirally, or of a single chamber only. There is 

 no " supplementary skeleton," and there is a large single or mul- 

 tiple "general aperture" to the shell. The apparently simplest 

 form of the Globigerinida is Orbulina (fig. 22), in which the test 

 has the external form of a single globular chamber, the walls of 

 which are perforated by a double series of foramina, large and 

 small. It seems more than doubtful if the test, as represented 

 in fig. 22, possesses any "general aperture." Doubt has been 

 thrown upon the true relationships of Orbulina by the discovery 

 that in many specimens, especially in those of small size, the 

 apparently monothalamous sphere contains in its interior a young 

 Globigerina-shell, attached to the inside of the wall by slender 

 spicules. It has therefore been held that Orbulina is really a 

 form of Globigerina in which the last chamber includes all those 

 previously formed. The genus Orbulina has a world-wide dis- 

 tribution at the present day, and its earliest representatives appear 

 in deposits as old as the Trias (Rhsetic). The genus Ovulites, 

 formerly placed in the immediate neighbourhood of Orbulina, ap- 

 pears, according to recent researches, to be in reality a Calcareous 

 Alga. The only other member of the Globigerinidce which requires 

 notice is the type-genus Globigerina itself (fig. 29, k), in which the 

 test is polythalamous and coarsely perforated (fig. 20). The cham- 

 bers are few in number, and globose in form, and are usually arranged 

 in a turbinate spiral, thus resembling the shell of a Rotalia. The 

 chambers do not communicate with one another directly, but each 

 opens by a special aperture into a deep central or umbilical depres- 

 sion. In pelagic forms (as also in Orbulina) the test, when perfect, 

 appears to be covered with long and extremely delicate spines. 

 Globigerina dates from the Trias, and is extremely abundant. It is 

 of special interest, as being the principal constituent of the " ooze " 

 (fig. 27) found at great depths in the larger oceans at the present 

 day ; while its shells form an equally large portion of the White 

 Chalk (see p. 119). 



In the large and important family of the Rotalidce, the test is 

 typically composed "of a succession of coarsely porous or globiger- 

 ine segments, arranged in a turbinoid spire, and communicating 

 with each other by a crescentic aperture situated at the junction of 

 the septal plane with the free surface of the convolution " (Car- 

 penter). The segments of the shell are typically so coiled that the 

 whole of the chambers are exposed to view on the upper surface, 

 whereas on the under side only the last convolution is visible, differ- 

 ent types varying as to which side of the shell is the more convex. 

 The shell-structure may be simple, but in other cases the test is 



