THE FORAMINIFERA 89 



part of the test already formed were to rotate on its axis of con- 

 struction through a definite angle in the interval between the 

 formation of one chamber and the addition of its successor. In 

 the genus Triloculina the rotation is through (approximately) one- 

 third of the circumference, in Quinqueloculina through two-fifths, 

 and the chambers are disposed in three and five radii respectively. 



In these genera the width of the chambers is, moreover, less 

 than in Biloculina ; and in Triloculina three, in Quinqueloculina five 

 chambers are exposed, at any given stage of growth on the contour 

 of the test. 



We may now turn to the phenomena of dimorphism as pre- 

 sented by members of this family. 



In representatives of all the genera included in the list on 

 p. 77, a well-marked difference has been shown to exist in the 

 size of the central chamber in the two forms of the species. Thus 

 in Biloculina depressa the diameter of the megalosphere (M) x has 

 been found to vary from 200 to 400 /*, and that of the micro- 

 sphere (m) 1 from 18 to 25 p (Fig. 24). 



In B. ringens the contrast is not so great (M = 54 /*, m = 20 /A). 

 In Triloculina M = 204 & m= 18 p. In Sigmoilina M- 96-150 ft, 

 m- 27-3 6 i*. In Adelosina M= 90-330 /*, m= 18 p. In Idalina 

 M- 180-440 fly m = 12 p. In Massilina a well-marked difference 

 is said to be present, though the actual dimensions are not 

 recorded. 



Turning to the plan of growth, in Cornuspira and Quinquelocu- 

 lina the tests are uniform, i.e. they are arranged on the same 

 plan throughout, from the part which immediately succeeds the 

 central chamber to the end of the test ; and this is the case in 

 both forms of the species. Massilina has biformed tests in both 

 megalospheric and microspheric forms, the earlier chambers being 

 arranged on the quinqueloctiline plan, and the later on the spiro- 

 loculine. But in several of the other genera a marked contrast is 

 found in the arrangement of the chambers in the megalospheric 

 and microspheric forms. In the species of these genera the tests 

 of the megalospheric forms are, for the most part, uniform, the 

 arrangement characteristic of the genus being followed throughout 

 the growth of the test, while the tests of the corresponding 

 microspheric individuals are bi- or tri-formed, the plan of growth 

 of the chambers changing once or twice before the test is 

 complete. 



Thus in many species of Biloculina the arrangement of the 

 megalospheric form is biloculine throughout (Fig. 24, a). In the 



1 It will be convenient to use the letters M and m to indicate the diameters of the 

 megalosphere and microsphere respectively, the "diameter" being taken to imply, 

 when the central chamber is not spherical, the mean between the long and short 

 diameters. 



