2 DE. CAEPENTEE’S EESEAECHES ON THE E0EA3IINIEEEA. 
this type, to the greater mdividualization 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 
detailed comparison of the stracture of two generic forms, which, whilst they so fa 
agree in general plan of growth as not only to have been ranked by M. n Okbioxt side 
by side in his order Silicostegues, but to have been placed by other systematists m close 
apposition, differ in the most marked manner as to aU the prticulars just enumerated. 
Both these types are of pecuUar interest :-the first, Pen^ophs, on accoim of the very 
wl range ofaariation it presents, which has led to the establishment of three „ 
apparently distinguishable by weU-marked differences in conformation, upon what I feel 
srtisfled wUl prove to be but individual modifications of one and the same ^eoific type, 
—the second, Operculma, as being the nearest existing representative of and 
consequently as affording not merely the key to the elucidation of the 
also tie 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 inore 
ritricted than in the preceding case, still quite sufficient to justify a large multiphca- 
tion of species, in the estimation of those who do not practise that extended method ot 
compai-ative inquiry, on the importance of which I have dwelt m a ^ 
74) With the latter of these genera, as also with NummuUtes, I shall p 
the genus AmpUstegina is closely allied; although M. d’Oebig^ misled by 
want of symmetry and by the alternation in the disposition of the chambers which are 
exhibited^ certain forms of that type, has placed it in a different order, Entamaegt^. 
For I shall have to show that a gradational variety in this respect, ending m comp e e 
symmetry, may coincide with such a uniformity in general stracture, that even a very 
decided departure from symmetry must be regarded as a character of little ralue m 
classification, compared with agi-eement in the organifaion of the shell and in those 
pecuUarities in the conformation of the animal which are indicated by it; and fuithei 
that a most marked difference in degree of organization exists between two species of 
AmpMdegim, 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 Histon/.-The genus PeneropKs was first instituted by Mohtfoet* to distinguish 
a peculiar type of minute polythalamous sheUs, which had been previously described and 
figured by Fichtel and MoLLf, and had been ranged by them with numeious otheis 
Idex the comprehensive designation Nmtilus; and Montfoet correctly mdicates its 
distinctive character, as “bouche de toute la longueur de la base, et percee 
par une file des pores,” though he seems to have very erroneously interpieted the sig 
. Bmrox de SoMlxi, 1802-1806 ; MoUasqaee, tome iv. p. 1. pl. 43 ; and Oonchyliologie Systamatique, 
1808,p.258. , .r. r. 
t Testacea Microscopica, Vindob. 1798*.1803, p. 91. tab. 16. figs, a, 
