THE FORAMINIFERA 121 



give rise to the deep pitting of the surface from which the genus 

 is named. Except in the early stages of growth there is no sub- 

 division of the test into definite chambers. The protoplasm is 

 contained between the laminae, and in irregular spaces which 

 occupy the axes of the branches ; it communicates with the 

 exterior by the numerous perforations in the laminae, and at the 

 ends of the branches where the axial spaces open widely. Their 

 mouths are often beset with sponge spicules, which appear to be 

 used as a temporary scaffolding for the support of the extended 

 pseudopodia, in advance of the proper wall. 



At the base, however, in contact with, or close to, the object 

 to which the Polytrema adheres, a spiral group of chambers is found 

 the initial stage of the test (Fig. 51, a, R, and b and c). These 

 initial chambers have the thick coarsely perforated walls, the 

 abundant chitinous element, and the spiral arrangement character- 

 istic of the order Rotalidea. 



In ten specimens of the megalospheric form, I find that M varies 

 from 110 to 29 /A, its average value being 51 p. In these 

 specimens there are generally three chambers, following the 

 megalosphere, arranged in a simple spiral (Fig. 51, c); the fourth 

 chamber usually communicates by apertures with two or more 

 chamber's, and after this the arrangement becomes more and more 

 irregular, all distinction between the chambers and connecting 

 passages is gradually lost, and the laminate structure of the test is 

 attained. 



I have not happened to meet with specimens of the microspheric 

 form, but this has been described by Merkel (23) l who finds that 

 the microsphere (size not given) is succeeded by a regular spiral 

 of some eleven chambers, before the chambers assume the irregular 

 transitional character. 



In some cases the spiral of initial chambers is separated from 

 the supporting object by a layer of small chambers of the irregular 

 transitional form, and Schlumberger (56) has found in sand from 

 the Azores small examples of the megalospheric form as free 

 globular tests, consisting of the large initial chambers invested on 

 all sides with a layer of small ones. It is evident, therefore, that 

 Polytrema may pass through a more or less prolonged period of 

 free life before it becomes adherent. 



Merkel found the nuclear condition to agree with that of 

 Polystomella, three examples of the megalospheric form containing a 

 single large nucleus lying in the megalosphere or an adjoining 

 chamber, while in one of the microspheric form four nuclei were 

 counted. 



1 In the megalospheric form, Merkel describes the megalosphere as communicat- 

 ing directly with some three of the surrounding chambers a condition which I have 

 not met with. 



