On Dimorphism in Alveolina boscii Defr. sp. By F. Chapman. 153 



tively higher and shorter, and consequently the chamberlets are 

 elongated in a vertical direction, or coincident with the minor axis 

 of the test. This peculiarity of its internal structure is seen ex- 

 ternally in the general shape, which is constant throughout the 

 growth of the foraminifer ; as will be readily recognised from an 

 inspection of the two photographs (Plate II. figs. 2, 3). The 

 lengths of the tests in forms A and B, of which slightly enlarged 

 photographs are now given, measure 7*5 mm. and 18*25 mm. 

 respectively. 



Concluding Observations. — It is already well recognised that, 

 whilst almost every type of rhizopod shell can be readily referred to 

 the asexual stage A, the alternating sexual stage, characterised by 

 the microsphere, is undoubtedly rare, and often extremely so. 

 Schaudinn, Lister, and others, who have contributed so much to 

 our knowledge of the life-history of this group, conclusively show 

 that the megalospheric form is the stage fitted for a quiescent- 

 conditioned reproduction of the species, and that the megalospheric 

 form can repeatedly give rise to other asexual, megalospheric 

 individuals. When, however, non-related individuals of the same 

 species are introduced, the zoospores of different parentage can 

 conjugate, and result in the production of microspheric examples. 



In the case of Alveolina the same rule holds good, and in a 

 very marked degree, for individuals of the melagospheric form 

 occur out of all ordinary proportion to the microspheric form, 

 which, as has already been shown, is known for certain only in 

 two solitary instances, one as a fossil, the other as a recent form ; 

 although it is probable that in some instances the long slender 

 tests of the microspheric shell of the living species may have been 

 passed over unnoticed as merely abnormally elongate examples. 

 Apropos of the last remark, it is of interest to note that W. B. 

 Carpenter, in his " Introduction,"* says : " The length of the longest 

 complete specimen in my possession is • 35 of an inch, but I 

 have a specimen whose shape is nearly cylindrical (the A. quoyii 

 of d'Orbigny), which, though incomplete at one end, measures 

 0'50 of an inch." The A. quoyii\ referred to by Carpenter is, in 

 all probability, another instance of form B, as may have been 

 Carpenter's own imperfect specimen. 



* Introduction to the Study of the Poraminifera, 1862, p. 99. 

 t Ann. Sci. Nat., vii. (1826) p. 307. 



April 15th, 1908 M 



