14 DE. CAEPENTER’S EESEAECHES ON THE EOEAJMINIEEEA. 
authors to differ from NummulUes in respect of its flatter form, the comparative short- 
ness of its spire, the smaller number of chambers, and the non-embrace of its convota- 
tions ; as also in absence of that narrowing of the later convolutions which leads to the 
entire closure of the spire in Nummulites. The genus Operouhna is said (loc. eit.) to 
be distinguished from Nummulites only by the depression of its form, the smaU number 
of its coelutions, and the rapid increase in breadth of the last whorl, which seems 
constantly to remain open. The genus Assilina, having no sufficiently distmctive 
characters, must (according to these able investigators) be merged 
is obvious, from these remarks, that MM. d’Aechiac and Haimb had fully recognmed 
the two fundamental errors of M. d’Oebight’s deflnition of Opermlina- vis. his state- 
ment that the convolutions are non-embracing, and his description of the foim of the 
aperture as triangular. And although Mr. Caetee* does not allude to the disciepancy 
between the structure of the recent type described by him and the generic definition o 
M d'Oebigst, yet it is evident that in assigning to it the designation Operouhna he was 
guided rather by its general resemblance to the fossils on vvhich that genenc name had 
Len conferred, than by the conformity of its structure to M. nOEBiGNTS definition. 
The errors of that definition are easily accounted for ; since it is only by such an exaim- 
nation of thin transverse sections as M. d’Oebigny (I have reason to beUeve) nevei made, 
that the fact of the earlier whorls being really embraced by the later, notwithstandmg 
the apparent freedom of the latter, can be substantiated ; and the determination of the 
form of the apertui-e is very likely to be erroneously made, when the exammation is 
limited to fossil specimens, in which it is frequently but very indistinctly traceable. 
141 The minute structure of this type has been already investigated by two exceUent 
observers. Under the designation of “ an undescribed species of Professor 
WiLLiAMSOuf has given an account of the structure of a shell abounding m theMa a 
sand, which, not merely from his figures and descriptions, but from a companson of the 
specimens which he has kindly enabled me to make, I know to be one of the smaUer 
forms of the Philippine Operoulim. Mr. H. J. Caetee$ has still more minutely described 
the organization of an Operculina which he obtained in great abundance on the south- 
east coast of Arabia, the shells coming up attached to the grease of the sounding lead 
from sandy bottoms of between ten and twenty fathoms depth; and of the identity o 
this with a larger form of Philippine Operculina I am enabled to speak, from companson 
of specimens which Mr. Caetee has obligingly transmitted to me. I find the descrip- 
tion of each to be in the main correct so far as it goes, but to be defective in some 
essential particulars. Thus Professor Wiluamsoh does not notice the canal-system 
except in the mai-ginal cord ; and Mr. Caetee, though he has more fully desciibed the 
canal-system, has not only missed one important part of it, but has misapprehended t le 
structure of the marginal cord. The investigations of each, moreover, have been limited 
* Ari-nals and Magazine of Natural History, Sept. 1852. 
t Transactions of Microscopical Society, Eirst Series, vol. iii. p. 112. 
+ Annals and Magazine of Natural History, Sept. 1852. 
