THE FORAMIN1FERA 95 



winds round half to three-quarters of its circumference before 

 opening into the first of the spiral series of chambers. As 

 Rhumbler has shown (35) the walls of the central chamber and 

 the beginning of the spiral passage are traversed by minute radial 

 perforations, so that the test formed in the earliest stage of the life 

 of this form of Peneroplis is perforated, though the walls formed 

 subsequently are, at any rate usually, imperforate, in accordance 

 with the rule in the Miliolidea. 1 



The microspheric form of Peneroplis is very scarce in the 

 material which I have examined. Among 1000 specimens I have 

 met with only five. Its proportion to the megalospheric form 

 appears, however, to vary in different localities. Thus, in a 

 sample of sand from the Maldive Islands, dredged in 47 fathoms 

 by Mr. J. S. Gardiner, the proportion is 3 to 108, or 1 to 36, 

 which is about the same as obtains in Polystomella. On the other 

 hand, in a batch of 480 specimens from Watson's Bay, I failed to 

 meet with a single microspheric form. The values of m in my 

 specimens are 17, 18, 19, 22, and 24 p.. On comparing these with 

 the values of M, it will be seen that the megalosphere may, in 

 some cases, fall below the microsphere in size. The two forms 

 are, however, sharply separated by the fact that, as in Orbiculina 

 and Orbitolites, the spiral passage is absent in the microspheric form 

 (Fig. 29). 



Orbiculimi. All the forms of the genus are included in a single 

 species, 0. adunca (F. and M.). Its distinctive features are the 

 subdivision of the chambers into chamberlets, and the equitant 

 character of the chambers in the earlier convolutions, giving rise 

 to a prominent umbo at the centre of the flattened test. It 

 affords a good example of the mode of occurrence of variation 

 in one of the Foraminifera with a definitely symmetrical test. 



Although the tests present themselves under a great diversity 

 of external shape, the variations from the normal are limited to 

 certain well-marked and definite lines. Fig. 30 shows the main 

 varieties, as they are represented in a sample of ballast sand from 

 the West Indies, kindly sent to me by Mr. F. W. Millett. 



The youngest tests are uniformly nautiloid (Fig. 30, a and b), 

 the chambers succeeding one another in a closely wound spiral. 

 As the sections represented in Fig. 31 show, the chambers are 

 elongated transversely to the course of the spiral ; hence we may 



1 The tests of Peneroplis present throughout their growth a surface pitting, which 

 is usually shallow, but may be so deep as to amount very nearly to perforation. 

 Indeed I am not convinced that some forms are not completely perforated in the 

 later as well as in the initial chambers. As we shall see below, the central chamber 

 and spiral passage of the megalospheric form are perforated also in Orbiculina and 

 Orbitolites marginalis. There appears therefore to be no justification for the separa- 

 tion of Peneroplis from the other genera of this family, as proposed by Rhumbler, 

 on account of its supposed peculiarity in this respect. 



