26S FAMILY NUMMl-LTNIDA. 



harder tliau the rest, stand out in greater relief; and thus it happens that the septal hands and 

 tubercles, which maybe scarcely distinguishable on some specimens, become very conspicuous 

 on others, although the structure of the shell may be precisely the same in the two cases. 

 Not only are the septal bands of the regular spire thus brought into view where this remains 

 unconcealed by subsequent overgrowth, but those alar continuations of them may be rendered 

 visible superficially, which, in the typical Nummulites, extend themselves from tlie last turn 

 of the spire over the whole of each lateral surface ; and thus those surfaces may come to be 

 marked by striae, which sometimes directly converge from the margin towards the centre, 

 sometimes follow a course more or less sinuous, sometimes remain distinct from each 

 other through their whole length, and sometimes divaricate and inosculate more or less 

 freely so as to form an irregular reticulation. — From what has been stated of the diversities 

 of form among Nummulites, it will be readily apprehended that their margin will some- 

 times be rounded or obtuse, sometimes thinned away to an acute edge. In adult specimens 

 it seems to be always smooth, not being marked by those furrows wjiich wc shall find (as in 

 Opereidina) to traverse the surface of the " marginal cord" of the inner whorls ; and it is 

 affirmed by MM. D'Archiac and Haime that this type is distinguished by the entire closure 

 of the last chamber, since, even in the best preserved specimens, they have not been able to 

 distinguish that fissure between the inner margin of the septum and the outer edge of the 

 preceding whorl whicli elsewhere constitutes the passage of communication between adjacent 

 chambers. To this point I shall presently more particularly advert (^ 466). 



458. Internal &fn/efurr. — For the satisfactory investigation of the internal structure of 

 Nummulites, it is requisite to make thin sections in various directions, capable of being viewed 

 under the microscope by transmitted light, and also to break the shell in various modes, so 

 as to obtain fragments which may be examined by reflected light.* Valuable information is 

 also furnished, especially in regard to the distribution of the canal-system, by the siliceous 

 casts of their internal cavities which have been occasionally preserved to us whilst the shells 

 underwent disintegration. — The indications afforded by these methods are such as to leave 

 no doubt that Nummulina very closely accords with OjiercuUna in the most essential features 

 of its internal structure ; and it will therefore be unnecessary to do much more than indi- 

 cate the differential characters by which the former type is specially distinguished. 



459. When wc examine the spire of a Nummulite as laid open by a section through the 



of Kelat and Scinde ; sliowing that much dependence must not be placed on the puucta or septal lines 

 for specific distinetiou." (See also Parker and Rupert Jones, lxxviii and lxxx i). 



* Nummulites are generally most disposed to split through the median plane, so as to lay open 

 the cavity of the spire. This property, which has been noticed in all ages, is due partly to the larger 

 jiroportiou of cavitary spaces which occup}' that plane (as seen in Plate XVIII, fig. 9), and partly to 

 the inferior density of that portion of tlie shell— the marginal cord — which hounds the edge of the 

 spire. If a Nummulite be lieated in a flame, or over the fire, it will often split into two halves 

 through the median plane, if it be thrown into cold water or be struck lightly on its edge with a 

 hammer : and this it will he more disposed to do, in proportion as it has been but little changed by 

 fossihzation, and has its cavities free from mineral infiltration. 



