i 



of the Shell of Operculina Arabica. 165 



. 159), which I shall here insert, with Dr. Carpenter's remarks, 

 as the whole appears in the ^ Quart. Journ. Geol. Soc.' vol. vi. pt. 1 . 

 p. 28, for I have not Mr. Williamson's paper to refer to : — 



" Of the contained animal itself, which he obtained by dis- 

 solving 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. Williamson ; 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 ex- 

 actly correspond with the single perforations in the septa of 

 Nummulites. The animal of Polystomella is considered by Mr. 

 ^ illiamson to derive its nutriment from pseudopodia, which are 

 projected through numerous minute apertures over the whole 

 sui'face of the shell. He has not clearly traced these pseudo- 

 podia, however, into connection with the segments occupying 

 the interior whorls, which, like those of Nummulites, are in- 

 vested by those of later formation ; but he mentions (as Ehren- 

 berg 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 

 i the innermost convolutions. Mr. Williamson goes on to point 

 ' out, that to these pseudopodia must be attributed the deposition 

 ' of new matter upon that portion of the central nucleus which is 

 i not covered by the investing whorls ; and in this view he is in 

 ^ accordance with M, D'Orbigny, who, in his recent work, ' Sur 

 I les Foraminiferes Fossiles du Bassin Tertiaire de Vienne,' fully 



i recognizes the power of the pseudopodia to secrete the calcareous 

 covering. I may remark, that I cannot see how the investing 

 layers covering the disk of Nummulites complanata, and the other 

 I species of the same group, can be formed in any other way ; 

 I since in these the chambers are only marginal, the segments of 

 f the animal not extending over the disk ; and we have no reason 

 f to believe in the existence of any external mantle, spreading 

 I over the whole surface, whereby these investing layers could be 

 I formed." 



