32 proceedings of the geological society. [may 2, 



Orbitoides. 



In adopting the generic name whicli M. D'Orbigny has conferred 

 upon the fossil previously termed Niimmidites Mantelli (Quarterly 

 Journal of the Geological Society, vol, iv. p. 12), and in extending 

 it to several other species, of which some had been previously ranked 

 among Nmmnulites, and others among OrhitoUtes, it is proper for 

 me to state that I am unacquainted with the definition of the genus 

 proposed by that distinguished palaeontologist, which has not, so far 

 as I am aware, been yet made public. The institution of the genus 

 appears to me to be required for the reception of a number of fossils, 

 which my own inquiries have led me to recognize as presenting a 

 type of structure so dissimilar to that of either of the groups just 

 named, as well as to every other with which I am acquainted, as to 

 render their separate association desirable. My account of the genus 

 will be principally drawn from the species just named ; of which, 

 through the kindness of Sir Charles Lyell, I have been enabled to 

 examine many excellent specimens from the nummulitic limestone 

 of Alabama, whilst by the liberality of Sir Roderick Murchison I 

 have had the opportunity of examining specimens of what I have no 

 hesitation in regarding as an identical species from the nummulite 

 limestone of Cutch. 



The Orhitoides Mantelli is a discoidal body, sometimes attaining 

 the diameter of an inch or even more, and having a thickness of 

 about a tenth of an inch near the centre, but gradually thinning 

 away towards the edges. No traces of cells are visible upon its ex- 

 ternal surface ; but when it is split through the medial plane (which is 

 the case wdth many of Sir Charles Lyell' s specimens), the exposed sur- 

 faces present a close resemblance to the exteriorof anOrbitolite; having 

 numerous cells with rounded or somewhat oval orifices arranged in 

 regular concentric rows, as shown in fig. 31, which represents a small 

 portion of the interior structure disclosed by a fracture which has 

 laid open some of the cells, but has left others with their covers un- 

 broken. The resemblance of the surface of this central layer of cells 

 to that of OrhitoUtes complanata (fig. 30) will be apparent on a 

 comparison of the figures ; and there is also a great similarity in the 

 aspect of the cells, when seen in a section parallel to the surface 

 (fig. 21), to that of the cells of the recent Orbitolites of New Holland, 

 when crossed b}^ a section taken in a similar direction (fig. 26). But 

 these resemblances end ; for we shall find that both in the structure 

 of the central layer itself, and in the addition of the crust on either 

 surface, which is totally unlike the two layers of ovoidal cells on the 

 exterior of the Australian coral, Orbitoides is extremely different from 

 Orbitolites. These differences are made evident by examining a sec- 

 tion of this fossil made perpendicularly to the surface (fig. 20) ; for 

 we then see that the cells or chambers of the medial plane form but 

 a single layer {a, a) ; and that this is covered-in, above and below, 

 by a thick crust (5, h), which is itself composed, like the shell of 

 Nummulite, of several layers with intervals between them. These 

 layers, however, when carefully examined, are found not to possess 

 the continuity of those formed by the investing whorls of Nummu- 



