4G8 ORIGINAL ARTICLES. 



possesses all the attributes of the body of which it is an extension, 

 and can maintain its existence With 1 equal readiness, either in a se- 

 parate state or in continuity with the stock of which it is an offset. 

 Although, therefore, there are certain types of Foraminifera in which 

 such offsets appear invariably to separate themselves before the con- 

 solidation of the shell, so that the original body never adds to the 

 number of its segments, and the shell remains " monothalamous," — 

 whilst there are others in which they ordinarily remain in connection 

 with the original stock, so as progressively to augment the number 

 of the segments and of the chambers of the " polythalamous " shell, 

 often to an indefinite extent, — I cannot see any such difference be- 

 tween the physiological conditions of the newly-formed segment in 

 the two cases, as would be required to justify the erection of the 

 Monotlialamia into a distinct order. Moreover, we find that each of 

 these groups, as ordinarily constituted, contains forms which in prin- 

 ciple should rank with the other. Thus the continuous spiral shells 

 which are known as Spirillince or Cornuspirce, having their cavities 

 undivided by septa, are always ranked amoug Monotlialamia ; but as 

 they have the capacity for indefinite extension, which is characteristic 

 of the Polythalamia, they need nothing but segmental division to turn 

 them into Rotaliw or Spiroloculince. Hence, such shells though 

 actually monothalamous, are potentially polythalamous; and to rank 

 them with Gromice, Lagence, or Orbulince, whose increase can only be 

 effected by the complete detachments of the superfluous segments of 

 sarcode, and by the formation of new and independent envelopes for 

 these, — the enlargement of their shells being forbidden by their 

 shape, would be antagonistic to the very principle on which the dif- 

 ferentiation is based. I have recently been investigating another 

 type, not until lately ranked among Foraminifera, which presents a 

 condition of precisely the converse nature. In Dactylopora and Aci- 

 cularia (as I shall more fully explain in my forthcoming Monograph), 

 we have composite organisms of definite form made up by the aggre- 

 gation of chambers which have no internal communication with each 

 other, each being as distinct from the rest as the chambers of a heap 

 of Lagence, and being only united by external adhesion. Such or- 

 ganisms, therefore, although actually polythalamous, are essentially 

 monothalamous; since the sarcode-body, contained within each cham- 

 ber, is as independent of the bodies enclosed in the neighbouring 

 chambers, as it would have been if these chambers had been alto- 

 gether disconnected. Again, there are certain Polythalamia, the 

 successive chambers of whose shells, although formed by continuous 

 gemmation the one from the other, are so slightly connected as to 

 be easily separable by accidental violence, and of which the animals 

 can maintain their lives just as well when they are thus broken up 

 into distinct segments as when retaining their original continuity ; 

 such, again, may be regarded as potentially Monothalamous ; and 

 the fact that the segments of sarcode, as they were successively 

 budded off from the stock, formed their shelly investments before, 



