GENUS NUMMULINA. 265 



shell, resembling those wliich occur in many recent types of Foraminifera ; and a more extended 

 acquaintance with the latter, and particularly the study of ( 'i/doch/peiis (xiv), satisfied me of the 

 correctness of their interpretation of the appearances by wliicli M. de Keyserling, MM. Jolj" 

 and Leynierie, and I myself had been originally misled. The error has l)cen endorsed, however, 

 by the high authority of MM. D'Arehiac and Haime (i) ; and, though I shall have to revert to 

 this subject more fully hereafter (^ 463), I may here remark that the contrast between the 

 transparent semi-crystalline aspect of the non-tubular portions of the shell, and the brown 

 semi-opacity of the tubular portions, is so striking in many Nummulites, whilst the limits 

 between the two are often so sharply defined, that the misinterpretation of these appearances 

 by such as have not familiarised themselves with analogous appearances among recent Fora- 

 minifera is almost inevitable. 



455. In my own Memoir, which was for the most part restricted to an account of the 

 structure of a single species, iV. lavif/ata, attention was for the first time drawn to the double- 

 ness of tlie septa that divide the chambers, and to the existence of a set of interspaces 

 between their laminoe, communicating with passages excavated in the walls of the chambers ; 

 to the minute tubularity of the shell forming the spiral lamina, and to the coarser tubularity 

 of that which forms the marginal cord ; as well as to the various structural features which 

 indicate the conformity of Nummulites to the general type of Nautiloid Foraminifera, as 

 elucidated by Prof. Williamson's observations on Polystonella crispa : and I further drew 

 attention to the varieties in the mode in which the alar prolongations of the septa are 

 disposed on the surface of the previous convolutions, as likely to afford characters of value 

 in the differentiation of species. — Not long afterwards (xviii) Mr. Carter published the results 

 of his inquiries into the structure of the allied sub-genus Operculina, which led him both to 

 confirm and to extend my discovery of the canal-system in Nummulite; and having examined 

 specimens of Nummulite with the additional light thus obtained, he saw enough to satisfy 

 him of the essential similarity of the distribution of the canal-system in the two types. The 

 elaborate monograph (i) of MM. D'Arehiac and Haime, which appeared a few years later, 

 brought together every discoverable notice of this type from the earliest times, giving a con- 

 nected history of the progress of knowledge respecting it, of which I have freely availed 

 myself in the preceding outline. They embodied also the most important parts of the 

 descriptions of its structure which had been given by those who had investigated it ; and to 

 these they added some new facts of importance, by following out the modifications which the 

 fundamental type undergoes, especially in regard to the course of the alar prolongations of 

 the septa, — an inquiry which I was myself only prevented from pursuing by the want of the 

 requisite materials. We shall hereafter see (^ 462) that these modifications (to the importance 

 of which as differential characters I had specially directed attention) furnish the basis of their 

 division of the genius into subordinate groups. All the reputed species and their synonyms were 

 most carefully investigated by them ; and they succeeded in reducing a chaotic assemblage of 

 about a hundred and fifty* to an orderly arrangement of fifty-five. According to the ideas of 

 classification then prevalent, they were fully justified in the differentiation of the greater part of 



* According to the Authors ubove referred to, (wenlij-two of the best kuowii species of Num- 

 mulite figured in catalogues as mnety-eight ; of these, 5 had been placed in tvjo genera, 1 in 



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