G9() Transactions of the Society. 



interior iuvarialily presents the same general characters — a primordial 

 chamber of relatively small dimensions, and a curved shelly process 

 sprinr'ing from its base — apparently the incomplete septum of a 

 second segment ; the whole inclosed in a large " circumambient 

 chamber" (figs. 8, 11). I have not in any case observed the com- 

 mencement of the annular mode of growth characteristic of the mature 

 shell, until after the embryo has left the parent. Subsequently the 

 peripheral apertures form the connection with the first annulus of 

 chamberlets (fig. i)) ; and from this point it is easy to follow the 

 successive stages of the growth of the test. Fig. 10 is drawn from a 

 young specimen consisting of the embryo or " primitive disc " and 

 three annuH of chamberlets, the test laid open so as to show the 

 interior. 



One interesting point remains. There is no diflBculty, as has just 

 been remarked, in tracing the growth of the shell, by the addition of 

 successive annuli of gradually increasing thickness, until the full size 

 of the adult OrhitoUtes comj)lanata is reached. The relatively large 

 " primitive disc " remains a conspicuous feature throughout, as shown 

 in almost every published drawing illustrating the structure of the 

 complex type of the genus. But the adult specimens under notice — 

 that is to say, the parent shells — present no such feature. The draw- 

 ings, figs. 13, 14, represent horizontal and transverse sections of the 

 central portions of two of these large viviparous specimens, the 

 magnifying power employed being the same as in figs. 4-12. By the 

 horizontal section it will be seen that, in place of the " primitive disc," 

 the centre is occupied by a multitude of small chamberlets arranged on 

 no very regular plan ; and what is more remarkable is the fact revealed 

 by the transverse section, namely, that at its centre the adult test is 

 scarcely 1 /300 in. (0 • 08 mm.) in thickness, or only about one-third 

 of the thickness of an embryo of average size. The ' Challenger ' 

 specimens of the same form, such as I have examined, though more 

 stoutly built, show the same absence of a " primitive disc." In one 

 or two instances I have observed at the centre of the shell a small 

 convexity, not unlike the structure referred to in point of size and 

 outline; but further examination showed that in every case it 

 consisted of a labyrinthic mass of httle chamberlets, to all appearance 

 of exogenous growth. 



We are indebted mainly to the labours of two French naturalists, 

 ]\DI. Munier-Cbalmas and Schlumberger, for a knowledge of the 

 existence of a sort of *• dimorphism " amongst the Foraminifera. They 

 have shown that in certain families, perhaps in all, but notably in the 

 Miliolidae, each species presents itself in two forms ; one of which, 

 called by them " Form A," has a large primordial chamber and 

 consists altogether of but few segments ; whilst the other, " Form B," 

 has a small initial chamber, and the succeeding segments are relatively 

 numerous. Two possible explanations are indicated by the authors, 

 the one which they prefer is based upon the supposition that " each 

 individual passes tbrough two successive phases, the first of which would 



