298 FAMILY NUMMULINIDA. 



shown in Plate XIX, fig. 6*), as apparently to indicate the existence of this genus at the 

 Nummulitic epoch to which these casts are to be referred ; but, as will presently appear, 

 this cast may not improbably represent a portion of the median layer of Orhifoides. 



Genus VII. — Orbitoides (Plate XX). 



502. History. — The genus Orbitoides vidiS first instituted by M. D'Orbigny in 1847 for 

 the reception of a peculiar fossil brought from the United States by Sir C. Lyell, who had 

 noticed it ('Quart. Journ. of Geol. Soc.,' vol. iv, p. 12) under the name of Nmnmulites 

 Mantelli previously conferred upon it by its discoverer Morton. I have not, liowever, been 

 able to discover that M. D'Orbigny gave any definition of the genus previously to the pub- 

 lication of the second volume of his 'Prodrome de Paleontologie Stratigraphique ' (1350), in 

 which he characterises it by a portion of tlie description which he afterwards (1852) more 

 fully gave in his ' Cours Elementaire ' (torn, ii, p. 194). It was then first that he instituted 

 the order Cydostegues, in which he ranked Orbitoides in sequence to Cyclolina, Orbitolites, and 

 Orhitolina, distinguishing it from those genera by the following characters : — " Coquille 

 discoidale convexe des deux cotes, formee d'une seule rangee des loges autour du disque ; 

 test fortement encroutc exterieurement au milieu, et montrant soit des lineoles rayonnantes 

 soit des granulations." Some time previously, however, I had given (xii) a nearly complete 

 description of the Orbitoides Mantelli ; and had further shown that a form of which small 

 specimens had been previously described under the name of Orbitolites Prattii (probably the 

 " Discolit/ius, IV, a," of Fortis), as well as the Lycophris epldppium and L. dispansus of Sowerby 

 (' Geol. Trans.,' 2nd ser., vol. v, pi. xxiv, figs. 15, 16), were all referable to the same type. 

 The genus has been adopted by Mr. Carter, who, however, in the first instance (xix), mis- 

 conceived its true structure (affirming its mode of growth to be spiral instead of cyclical), and 

 who now (xxiii «t) rejects from it the very type on which the genus was founded, referring 

 it (on fallacious grounds, as I shall presently show) to the genus Orbitolites. The genus has 

 also been adopted by MM. D'Archiac and Haime (i), and by Bronn (xi), who, however, 

 re-names it Hymenocyclus, objecting to Orbitoides as a hybrid word. 



503. External Characters. — It is impossible to discriminate Orbitoides from Nummulina 

 by external characters alone ; for although the most characteristic forms of each type cannot 

 be mistaken for each other by any one who has familiarised himself with their respective 



* The very close conformity of this somewhat diagrammatic representation to Prof. Elirunberg's 

 figure, might suggest the idea of its having been derived from the latter. But the former had been 

 constructed by Mr. G. West, from specimens prepared by myself, long before the publication of Prof. 

 Ehrenberg's researches. 



t I think it well to state that Plate XX had not only been drawn, but printed, before I became 

 acquainted with ^Ir. Carter's later researches. 



