2 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



this type, to the greater individualization of the parts of the composite body, which in 

 the preceding must be looked upon rather as constituting one aggregate whole. 



120. I purpose in the present memoir to carry on this contrast, by presenting a 

 detailed comparison of the structure of two generic forms, which, whilst they so far 

 agree in general plan of growth as not only to have been ranked by M. D'ORBIGNY side 

 by side in his order Helicostegues, but to have been placed by other systematists in close 

 apposition, differ in the most marked manner as to all the particulars just enumerated. 

 Both these types are of peculiar interest : the first, Peneroplis, on account of the very 

 wide range of variation it presents, which has led to the establishment of three genera, 

 apparently distinguishable by well-marked differences in conformation, upon what I feel 

 satisfied will prove to be but individual modifications of one and the same specific type ; 

 the second, Operculina, as being the nearest existing representative of Nummulites, and 

 consequently as affording not merely the key to the elucidation of the structure, but 

 also the basis for the determination of the value of the reputed species, of that genus, 

 by the study of the range of variation which it presents ; this range being, though more 

 restricted than in the preceding case, still quite sufficient to justify a large multiplica- 

 tion of species, in the estimation of those who do not practise that extended method of 

 comparative inquiry, on the importance of which I have dwelt in a former Memoir 

 (^f 74). With the latter of these genera, as also with Nummulites, I shall prove that 

 the genus Amphistegina is closely allied ; although M. D'ORBIGNY, misled by the marked 

 want of symmetry and by the alternation in the disposition of the chambers, which are 

 exhibited by certain forms o f that type, has placed it in a different order, Entomostegues. 

 For I shall have to show that a gradational variety in this respect, ending in complete 

 symmetry, may coincide with such a uniformity in general structure, that even a very 

 decided departure from symmetry must be regarded as a character of little value in 

 classification, compared with agreement in the organization of the shell and in those 

 peculiarities in the conformation of the animal which are indicated by it ; and further, 

 that a most marked difference in degree of organization exists between two species of 

 Amphistegina, which so closely resemble each other externally that the young of one 

 may easily be mistaken for the adult of the other. 



Genus PENEROPLIS. 



121. History. The genus Peneroplis was first instituted by MONTFORT* to distinguish 

 a peculiar type of minute polythalamous shells, which had been previously described and 

 figured by FICHTEL and MoLLf, and had been ranged by them with numerous others 

 under the comprehensive designation Nautilus; and MONTFORT correctly indicates its 

 distinctive character, as " bouche de toute la longueur de la base, et percee serialement 

 par une file des pores," though he seems to have very erroneously interpreted the signi- 



* Burrow de SONNINI, 1802-1805 ; Mollusques, tome iv. p. 1. pi. 42 ; and Concbyliologie Systeinatique, 

 1808, p. 258. 

 f Testacea Microscopica, Vindob. 1798-1803, p. 91. tab. 16. figs, a, i. 



