GENUS ORBITOIDES. 299 



aspects, yet specimens are often met with in which all distinctive features are so completely 

 wanting, that nothing save an examination of their internal structure can enable their real 

 nature to be determined with certainty. The typical form seems to be a flattened lenticular 

 disk, as represented in Plate XX, fig. 1, which very closely resembles that of the thinner 

 Nummulites, except in the relatively greater convexity of the central region, — in this respect 

 corresponding closely with Cydodypnis (Plate XIX, fig. 2). This convexity sometimes 

 increases so remarkably that the central region becomes almost globular, whilst the peri- 

 pheral thins-out and becomes almost flat, as in the ibrm delineated by Sowerby (loc. cit.) as 

 Lycophris dispansits. In other instances, on the contrary, the convexity of the central is not 

 greater than that of the peripheral portion ; the form being then so regularly lenticular as 

 to afford no means of distinguishing such an Orhiloides from a Nummulite. Again, the disk, 

 instead of being lenticular, may be nearly as flat as a piece of money, being commonly of the 

 like proportionate thinness, but sometimes as thick as two or three ordinary coins laid one 

 upon another ; in either of these cases, however, there is usually a central tubercle that serves 

 to mark the type ; and the thinner forms are frequently reduced to extreme tenuity at their 

 margin, whilst the margin of the thicker is abruptly rounded-off with little or no previous 

 reduction. The thinner forms are often more or less contorted, and not unfrequently they 

 assume a regularly ephippial shape, as in \he Lycophris ephippimi of Sowei'by (loc. cit.). I am 

 disposed to think that such contortion is due to the inequality of the surface to which the 

 specimens were attached during life ; for in a specimen from Biaritz kindly presented to me 

 by Mr. S. P. Pratt, it was pretty evident that the organism had spread itself irregularly over 

 a rock, after the manner of an incrusting Zoophyte. — The only surface-marking that can be 

 distinguished, and this only in well-preserved specimens, is an irregular areolation, in the 

 midst of which rounded tubercles of variable size are often disposed. These tubercles are 

 sometimes of a more opaque white than the rest of the disk ; whilst sometimes, on the other 

 hand, they have an aspect of greater transparency. Not unfrequently they occupy the 

 centres of groups of areolse, which are quincuncially disposed around them, as is shown at 

 e, e, fig. 2, Plate XX. In the smaller and thinner specimens they are most commonly 

 absent, or, if present, they are comparatively few and insignificant. The presence of the 

 tubercles alone would not serve to differentiate an Orbitoides from a Nummulina ; but the 

 areolation, when distinguishable, is a positive character, since nothing like it is ever pre- 

 sented even by the " reticulate " Nummulites. 



504. Infernal Structure. — The disk of Orlj'itoides essentially is composed (Plate XX, 

 fig. 1) of a median plane of chamberlets arranged cyclically round a large central chamber; 

 and of numerous layers of flattened chamberlets, having neither regularity of form nor 

 systematic plan of arrangement, that are interposed between the multiple lamellre of shell 

 which enclose the median plane of chamberlets on either side. — Although the chamberlets of 

 the median plane are disposed in concentric annuli, yet these annuli are often incomplete, as 

 shown in Plate XX, fig. 1 ; thus bearing a much closer resemblance in arrangement to those 

 of Cycloclypeus (H 499) than to those of Orhitolitvs (t 171). There is so decided and constant 

 a difference as regards the form of the chambers between 0. Mantclll and 0. Fortinii, that, 

 until such a gradational series of connecting links shall be discovered as unites the similarly 



