52 OF THE FORAMINIFERA GENERALLY: 



individually small (Plate VII, fig. 18), yet their multiplication (which takes place proportionally 

 to the size of the chambers) renders the aggregate communication very free, as we see in the 

 deiidriftue variety, in which the pores are all gathered together, as it were, into one aperture 

 (Plate VII, fig. 1 ) ; and the contrast is very marked between the large si^e of this, and the extreme 

 narrowness of the fissure which forms the only constant communication between the chambers 

 of OpercuJina. The like contrast, in regard to the connection of the successive segments, exists 

 between Orbiculina, the spiral type of which may be described as a Pnirroplis whose principal 

 chambers are partially subdivided by transverse partitions, and Hcferosfet/ina, which bears the 

 like relation to Operciilina. And a no less striking difference exists, in this respect, between 

 Orbitolites, which is the most developed type of the imperforate series, and Ci/e/oc/ypei/s, which 

 holds a like position in the perforated ; the communications between the successive annuli in the 

 former, and between the outermost annulus and the exterior, being so numerous and free, as to 

 amount in the aggregate to a large apertural area, whilst in the latter they are so much more 

 restricted as to be not readily discoverable. This tendency to a more complete separation of 

 the segments sliows itself yet more strongly in regard to the transverse subdivision of the 

 principal chambers in Orbiculuia as compared witli Ili'tcroxti'gina, and in Orbitolite-s as com- 

 pared with Cydoclypeus ; for in the two imperforate types we find this transverse subdivision so 

 far from complete, that it resembles that of a long dormitory in which the beds (arranged in 



rows on the two sides) are separated from each other by partitions 

 that extend from the walls towards the central line of the apart- 

 ment, but stop short of it, so as to leave a free passage from one 

 end of it to the other ; whilst in the two perforated types the 

 chamberlets of each row are entirely cut off from each other 

 laterally, communicating only with those of the rows behind and in front of them. Thus 

 we see that, whereas in the imperforate shells the nutrition of the entire body is 

 derived from the alimentary materials obtained l)y the last segment alone, every segment 

 in the perforated shells is capable of obtaining, at an}' rate, a portion of its supply for itself; 

 and hence a much greater degree of individualization of the segments is possible in the 

 latter case than in the former. We shall find tiiis contrast most strongly marked when 

 we come to compare the calcareous skeleton of OrbifoNfe-s- with that of Cydoclypeus ; the 

 former being, as it were, a concretionary framework, which grows up in the midst of and around 

 the sarcode-body for its support and protection, isolating its parts from each other no more than 

 is required for that purpose ; whilst in the latter each segment and sub-segment has its own 

 distinct and complete envelope, which seems (as it were) to be moulded upon its shape, any 

 interspaces being filled in by the intermediate skeleton, whose intervention between the two 

 layers of the double partitions renders the isolation of the chambers yet more complete. 



65. Thus, then, we find that, alike in the intimate structure of the shell, in the presence 

 or absence of an " intermediate skeleton" and of a " canal-system" for its nutrition, in the com- 

 pleteness witli which each chamber is surrounded by its own proper wall, and in the degree 

 of its separation from adjacent chambers — all which features are as characteristic of every 

 individual portion of the shell as they are of the shell as a whole, and are evidently in intimate 

 relation with the physiological condition of the animal that inhabits it, — a very decided 

 differentiation may be established between the two series of imperforate and perforated 



