FORAMINIFERA. 131 



each other from the same end of the long axis, or alternately at the 

 two ends, or in cycles of three, and the one last formed more or 

 less completely embraces those previously formed. In Chilosto- 

 mella itself, the segments are oval, and each entirely envelops the 

 previous one ; whereas in Allomorphina, the chambers alternate at 

 three sides so as to leave portions of two, in addition to the last one, 

 exposed to view. The former genus is confined to the Tertiary 

 deposits, and the latter is Cretaceous and Tertiary. 



The great family of the Lageiiidce comprises " hyaline " or " vit- 

 reous " Foraminifera, with a calcareous shell, the walls of which are 

 pierced by numerous minute pores, and are usually more or less 

 strikingly thin and glassy. In the compound forms of this group 

 the successive chambers have their posterior walls formed by the 

 front wall of the preceding segment, so that the septa are always 

 single, instead of being double, and there is never any " inter- 

 mediate" skeleton. The family may be divided into two series, 

 Lagena itself being the type of the one, while Nodosaria is the type 

 of the other. In Lagena (fig. 29, /) the shell is simple, flask-shaped, 

 unilocular, with a single prominent aperture. The genus commen- 

 ces in the Silurian, with forms little different from, or identical with, 

 existing types, is further developed in the Secondary and Tertiary, 

 and is well represented at the present day. Polymorfihina (fig. 29,/) 

 is allied to Lagena, but it is multilocular, the chambers being usu- 

 ally arranged in a double series. It is represented in the Trias, 

 and survives under common types at the present day. In the 

 series of which Nodosaria is the type, we have perforate Forami- 

 nifera consisting of a succession of chambers, each of which is 

 essentially similar to a Lagena, arranged in a series, which is usually 

 nearly or quite straight, though sometimes spirally involuted. In 

 Nodosaria itself (fig. 29, g) the chambers are simple, and are dis- 

 posed in a straight line. It ranges from the Permian to the present 

 day. Dentalina, ranging from the Carboniferous onwards, is funda- 

 mentally like Nodosaria, but the shell is bent like a bow. Vagin- 

 ulina comprises forms similar to Nodosaria, but laterally compressed, 

 and begins in the Trias (Rhaetic). Marginulina (fig. 29, h) is slightly 

 curved, or is sometimes crosier-shaped, and also starts in the Trias. 

 Frondiadaria (fig. 29, i) has the shell flattened out and leaf-like, 

 and likewise makes its first appearance at the summit of the Trias. 

 Lastly, Cristellaria (with Robulina) comprises forms more or less 

 spirally inrolled or crosier-shaped, which extend from the Trias to 

 the present day, and have a very wide development both individu- 

 ally and specifically. 



In the family of the Globigerinidce, Dr Carpenter included all 

 those hyaline Foraminifera, in which the calcareous shell is perfor- 

 ated by large-sized pseudopodial foramina, but the group, as defined 



