748 
THE VOYAGE OF H.M.S. CHALLENGER. 
preceding it. The segments are very numerous and V-shaped or embracing. In the 
larger and more characteristic forms the final convolution becomes gradually more and 
more contracted at the peripheral margin, until eventually it ceases to be distinguishable, 
completely closing in the shell. The aperture is a simple arched or V-shaped slit at the 
inner margin of the final segment. The canal system is highly developed, and forms an 
important feature of the genus, resembling in the distribution of its parts that of 
Operculina and Cycloclypeus. 
It is exceedingly difficult, perhaps impossible, to separate the genera Operculina and 
Nummulites by any well or strongly defined characters, indeed the former is regarded by 
Parker and Jones as only a subgeneric modification of the latter type. 
With certain authors the explanate varieties of Nummulites, that is to say, those in 
which the outline of the successive convolutions is visible externally owing to the 
tenuity of the lateral flaps of the chambers, constitute a distinct genus or subgenus, 
for Avhich the d’Orbignian term Assilina is employed. 
The genus Nummulites is subdivided by d’Archiac and Haime 1 into six “groups,” 
based upon the condition of the surface of the test and the form of the alar extensions of 
the chambers, namely: — 1. Laves aut Sublaves; 2. Reticulata; 3. Subreticulcita ; 
4. Punctulatce ; 5. Plicata vel Striata; 6. Explanata. The same classification is 
followed by Hantken and Madarasz. 2 Parker and Jones regard the “granulate” and 
“ explanate ” sections as needless, and divide the series into'three catagories : — 1. Radiata; 
2. Sinugta ; 3. Reticulata . 3 The latest arrangement is that proposed by de la Harpe, 
who accepts Assilina as a distinct genus, and separates Nummulites into two primary 
groups — (A), Non-reticulate ; and (B), Reticulate; each of which is divided into 
“ Granulate ” and “ Non-granulate ” species. 4 
As already stated, living examples of the genus are comparatively scarce. Further- 
more, it is difficult in the present state of our knowledge to speak with certainty as to the 
area of distribution of the recent forms, inasmuch as some of the specimens which have 
been obtained from dredged sands are, there can be little doubt, derived from Tertiary 
deposits. Minute but well-characterised Nummulites have been found in the fossil • 
condition at intervals as far back as the limestones of the Carboniferous period; though 
it was not until the commencement of the Tertiary epoch that ariy extensive develop- 
ment of the genus took place. The limestones of the Eocene period, which form an 
important constituent of the mountain-ranges of Central Europe, Central and Southern 
Asia, and Northern Africa, are largely composed of Nummulitic shells ; and, though 
the genus is less prominent in the Oligocene and subsequent formations, it is present to a 
greater or less extent at almost every stage of .the Tertiary system. 
1 Descr. des anim. foss. du groupe nummulitique de l’lnde, 1853, p. 72. 
2 Katalog d. auf d. Wiener. Weltausstellung im Jahr.e, 1873, ausgestellten Nummuliten. . 
3 Ann. and Mag. Nat. Hist. 1861, ser. 3, vol. viii. p. 230. 
4 Etude des Nummulites de la Suisse, l ii:re partie, 1881, p. 62. 
