28 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 2, 



to have possessed, as it is by tteir agency that the gro\\1;h of the 

 shell is continued by the addition of new whorls. 



There is a very striking conformity between the structure of the 

 shell of Nummulites and that of Polystomella crispa, of which an 

 elaborate description has recently been given by my friend Mr.W. C. 

 Williamson (Transactions of the Microscopical Society, vol. ii. p. 159 

 et seq.). I would especially call attention to the following points 

 of correspondence. Mr. W. states that, after a variety of examina- 

 tions, he has satisfied himself " that each septum separating two 

 cells, consists of a double calcareous layer, the soft inhabitant of each 

 shell secreting its own share." He has noticed, like myself, irregu- 

 larities in the growth of the shell, which are totally inconsistent with 

 the idea of its having been formed by a single body in continuous 

 growth, like that of the testaceous Cephalopoda. " Sometimes," he 

 says, '' the cells continue gradually to increase in size with symme- 

 trical uniformity, when suddenly one of them is found to be arrested 

 in its development, not attaining to half its proper dimensions ; and 

 the new ones subsequently added, are regulated in their increase of 

 size, not by those which had previously attained to their full develop- 

 ment, but by the one which has been so stunted, from the contracted 

 form of which, the cells again continue to grow and increase in regu- 

 lar order ; only being thrown back, as regards their size, by almost 

 an entire convolution." Of the contained animal itself, which he 

 obtained by dissolving away the calcareous matter of the shell with 

 dilute acid, Mr. Williamson says, that it consisted " of a very thin 

 external membrane filled with gelatinous matter." " No trace of 

 minute internal organization, such as a specially located intestinal 

 canal, or ovaries, could be detected " by Mr. W. ; nor was he able 

 in any instance '^ to discover with certainty the presence of any 

 foreign bodies in their interior." The several segments are described 

 by him as connected by a series of prolongations, which pass through 

 the septa near their inner margins. The segments at first formed 

 have only single connecting necks ; but the number of these soon 

 increases, and the outer segments are connected by ten or more such 

 necks, which pass through as many distinct orifices in the septa. If 

 all these orifices were brought together on the central plane, so as to 

 coalesce into one, they would exactly correspond with the single per- 

 forations in the septa of Nummulites. The animal of Polystomella 

 is considered by Mr. Williamson to derive its nutriment from pseu- 

 dopodia, which are projected through numerous minute apertures 

 over the whole surface of the shell. He has not clearly traced these 

 pseudopodia, however, into connection with the segments occupying 

 the interior whorls, which, like those of Nummulites, are invested by 

 those of later formation ; but he mentions (as Ehrenberg had done), 

 that near the umbilicus they are projected in fasciculi ; and he states 

 that the surface of the central calcareous nucleus (which is formed 

 by a thickening of the walls of the smallest cells) is pitted by small 

 but deep depressions, which may be designed to facilitate the exit of 

 the pseudopodia from the innermost convolutions. Mr. Williamson 

 goes on to point out, that to these pseudopodia must be attributed 



