14 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



authors to differ from Nummulites in respect of its flatter form, the comparative short- 

 ness of its spire, the smaller number of chambers, and the non-embrace of its convolu- 

 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 Operculina is said (loc. cit.) to 

 be distinguished from Nummulites only by the depression of its form, the small number 

 of its convolutions, and the rapid increase in breadth of the last whorl, which seems 

 constantly to remain open. The genus Assilina, having no sufficiently distinctive 

 characters, must (according to these able investigators) be merged in Nummulites. It 

 is obvious, from these remarks, that MM. D'ARCHIAC and HAIME had fully recognized 

 the two fundamental errors of M. D'ORBIGNY'S definition of Operculina; viz. his state- 

 ment that the convolutions are non-embracing, and his description of the form of the 

 aperture as triangular. And although Mr. CARTER * does not allude to the discrepancy 

 between the structure of the recent type described by him and the generic definition of 

 M. D'ORBIGNY, yet it is evident that in assigning to it the designation Operculina he was 

 guided rather by its general resemblance to the fossils on which that generic name had 

 been conferred, than by the conformity of its structure to M. D'ORBIGNY'S definition. 

 The errors of that definition are easily accounted for ; since it is only by such an exami- 

 nation of thin transverse sections as M. D'ORBIGNY (I have reason to believe) never made, 

 that the fact of the earlier whorls being really embraced by the later, notwithstanding 

 the apparent freedom of the latter, can be substantiated ; and the determination of the 

 form of the aperture is very likely to be erroneously made, when the examination 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 excellent 

 observers. Under the designation of " an undescribed species of Nonionina" Professor 

 WILLIAMSON f has given an account of the structure of a shell abounding in the Manilla 

 sand, which, not merely from his figures and descriptions, but from a comparison of the 

 specimens which he has kindly enabled me to make, I know to be one of the smaller 

 forms of the Philippine Operculina. Mr. H. J. CARTER J 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 of 

 this with a larger form of Philippine Operculina I am enabled to speak, from comparison 

 of specimens which Mr. CARTER 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 WILLIAMSON does not notice the canal-system 

 except in the marginal cord ; and Mr. CARTER, though he has more fully described the 

 canal-system, has not only missed one important part of it, but has misapprehended the 

 structure of the marginal cord. The investigations of each, moreover, have been limited 



* Annals and Magazine of Natural History, Sept. 1852. 



t Transactions of Microscopical Society, First Series, vol. iii. p. 112. 



J Annals and Magazine of Natural History, Sept. 1852. 



