THE FORAMINIFERA 



91 



by the presence of a thin sandy layer on the surface of the test, which 

 none of the other species inhabiting the same locality possess (55). 



A difference in the arrangement of the chambers throughout growth 

 is also said to be found in Adelosina polygonia, Schlumb., but it is in part 

 of a different character. In this species the chambers are all arranged in 



Fio. 25. 



Sections of the test of Triloculina schreiberiana, d'Orb, x 66. a, the megalospheric form, 

 b, the microspheric form. (After Schlumberger, 57.) 



a single plane in the megalospheric form, and after a preliminary 

 quinqueloculine phase they are arranged in a single plane in the micro- 

 spheric form. So far the arrangement agrees with that of Biloculina. 

 But the chambers of Adelosina polygonia do not occupy a half turn of the 



Fio. 26. 



Adelosina polygonia. a, the megalospheric form ; b, the microspheric form, x about on 

 (After Schlumberger, 54.) 



spiral, as do the chambers in that genus, but only one-third or a quarter 

 of a turn. In the microspheric form each chamber occupies one quarter 

 of a turn of the spiral, and the tests are, in consequence, quadrangular in 

 outline. In the megalospheric form the chambers occupy each one-third 

 of a turn, and the resulting tests are triangular (Fig. 26). In about 1 per 

 cent of the individuals of this form, however (310 were examined), the 

 quadrangular arrangement was adopted in the last whorl (54). 



