A. EARLAND ON ORBICULINA ADUNCA. 91 



There must be a reason for this difference, and it appears to 

 me to be due to the condition and vigour of the animal when it 

 reaches that point in its life at which it becomes necessary to 

 abandon the nautiloid method of growth. The oral aperture 

 in Orbiculina, as in Peneroplis, consists of a series of openings, 

 more or less sieve-like in appearance, situated on the front of the 

 last formed segment. Now, as these openings are the only means 

 by which the sarcode body of the animal has access to the sur- 

 rounding water and ooze, from which it derives not only its food 

 but also the supply of carbonate of lime for the construction of 

 its shell, it follows that the greater the surface covered with these 

 oral apertures, the larger the supply of food and building material 

 obtainable, and consequently the more vigorous the after-growth 

 of the animal. Even in the nautiloid early stages it is evident 

 that there is some difficulty in obtaining a sufficient supply of 

 nutriment from the limited area of apertures open to the sarcode, 

 for there is a constant tendency to extend the oral surface by 

 elongating the V-shaped surface of the final segment in a back- 

 ward curve. No doubt a point is eventually reached when it 

 is impossible to obtain an adequate supply of nutriment to main- 

 tain this method of growth, and the variation then commences. 

 The stronger and more vigorous specimens, being able to build 

 more rapidly and to extend their segments over a longer front, 

 enlarge their feeding area immensely, and so obtain increased 

 vigour and building power, eventually attaining the discoidal 

 form, in which they may be said to have mouths all round them. 

 On the other hand, those individuals which are hampered at 

 the start by weakness and an inability to increase their feeding 

 area continue to grow in a weakly manner, and form the crosier- 

 shaped forms. 



I do not know whether any student of the Foraminifera has 

 ever put forward this theory of growth to account for the varia- 

 tion of form among the Peneroplidinae, but it appears to me to 

 obtain additional force from the occurrence of certain aberrant 

 and monstrous varieties which are to be found among discoidal 

 specimens. When an Orbiculina has reached the discoidal shape 

 accompanied by its annular method of growth, it is evident that 

 it can only increase its area of oral apertures by continuing its 

 growth in a fresh plane. Hence in some specimens we observe 

 what we may describe as flanges of segments thrown off at various 



