48 FORAMINIFERA 



of the species. Unfortunately the microspheric form is often 

 rare and the megalospheric form the common one. Acceleration 

 of development takes place in various groups so that there are 

 species which hold to ancestral characters and show several 

 developmental stages, whereas others are much more specialized 

 and skip or greatly reduce these stages. Also parallelisms, or 

 the development of similar structural forms in the adult, are 

 very common in the foraminifera. 



The simplest foraminifera are the Allogromiidae where the 

 test is either wanting or consists of a thin chitinous wall. This 

 latter may be more or less globular or elongate and open at both 

 ends. With the advent of the arenaceous or agglutinated test 

 on the outside of the thin chitinous layer, a structure of more or 

 less permanency is established, and in the strongly cemented 

 tests is capable of preservation in the fossil series. One of the 

 simplest of this form of test is that seen in the Astrorhizidae 

 (PI. 2) where there is a central chamber and irregular arms, the 

 whole test rather loosely cemented. Another simple form is the 

 agglutinated test open at both ends in the Rhizamminidae 

 (PI. 3), a form easily derived from such primitive forms as 

 Shepheardella in the Allogromiidae (PI. 1, fig. 9). The simple 

 single chambered forms of the Saccamminidae (PI. 4) may be 

 directly derived from such primitive forms as Allogromia (PI. 1, 

 figs. 6, 7). 



As the next simplest stage in development, there is the initial 

 chamber of proloculum followed by a long undivided tubular 

 chamber. This may be straight as in Hyperammina (PI. 6, 

 figs. 1, 2) or become coiled in various types of spirals as in 

 Aynmodhcus and its allies (PI. 9). The planispiral form is the 

 simplest of these and occurs in the different types, the 

 arenaceous in Ammodiscus (PI. 9, figs. 2-4), in Comuspira in 

 the imperforate calcareous group (PI. 20, figs. 1, 2), and in 

 Spirillina in the perforate calcareous group (PI. 39, fig. 1) . The 

 long chamber next becomes divided into divisions or chambers, 

 and the various families gradually make their appearance in the 

 fossil series. 



In the Textulariidae (PI. 11) , the early stages are planispiral, 

 but the later ones become twisted and elongate finally settling 

 down to a test with the chambers making a half turn of 180", 

 the aperture toward the central axis. From this type are derived 



