406 On Dimorphism in the Milioliuas. 



synonymy of Miliolina oblonga (pp. 7 and 8) sufficiently 

 supports in the direction of the views above referred to. 



The special biological value of the presence of either a large 

 or a small initial chamber in any Foraminifer was at first a chief 

 subject of study by De la Harpe and other observers above- 

 mentioned ; but completely definite results have not yet been 

 arrived at*. Specimens having a large primordial segment 

 or chamber {megasphere — for megalosphere) are classed as 

 " A," and the others (with a microsphere) are the " B " 

 forms. The late Ph. De la Harpe had noticed eleven such 

 pairs or couples of Nummulites in 1880—81 ; and since then 

 numerous other such twin forms among the Foraminifera 

 have been described. It has been thought by some that the 

 A forms are the young and B the adults ; others have re- 

 garded A and B as distinct species ; others have looked on 

 them as individual modifications of the same species; and 

 hypothetical views have been held as to one sort and not the 

 other being capable of producing germs, that the central por- 

 tion may have been absorbed and reconstructed, and so on. 

 At all events those Nummulites grouped under A are generally 

 " free-growing individuals, soon arriving at the limit of 

 growth" (P. & J., 1861), whereas the others (B) attain a 

 larger relative size. This variation in the individuals of one 

 species M. Munier-Chalmas at first (1880) termed " dimor- 

 phism," with a different application to that of Dimorphina 

 (d'Orbigny, 1816), and " dimorphous " (P. & J., 1860) and 

 " trimorphism " (P. & J., 1863), which have reference to 

 successive stages from one style of growth to another. The 

 later expositions, however, by Munier-Chalmas and his 

 colleague of the growth of the Miliolidse (as noticed above) 

 may be said to give the term its full meaning as to the 

 passage-forms in individuals altering their plan of growth, 

 with or without reference to the primordial chamber. As 

 the dimorphous forms of Miliolina, Peneroplis, Lituola, Tex- 



* From Mr. J. J. Lister's " Contributions to the Life-History of the 

 Foraminifera" (Proceed. Roy. Soc. vol. lvi. no. 337, September 1894, 

 pp. 55-160), and a recent letter, we gather that in Polystomella the micro- 

 spheric form contains many comparatively small nuclei, while the mega- 

 lospheric form has a single large nucleus. Also that the niegalospheric 

 form of a species is much more numerous than the microspheric. While 

 in some instances the parent of megalospheric young has been found to 

 be microspheric, in others it was megalospheric; and in one species 

 (Orbitolites complanata) both the megalospheric and microspheric forms 

 have been found producing megalospheric young. In Polystomella, after 

 certain nuclear changes, the protoplasm of a megalospheric shell produced 

 flagellate swarm-cells (isospores) ; and similar bodies were seen escaping 

 from a whole shell. Mr. Lister suggests that the two forms of Foramini- 

 fe ra represent recurring generations. 



