of the Shell of Operculina Arabica. 175 



ried on in Orbitolites, where the mass is made up of spheroid or 

 ovoidal cells : for if the nearly flat Indian species, which has a 

 papillary eminence in the centre of the convex side, be rubbed 

 down, the latter presents a ramification of transparent substance 

 like that filling the septal spaces of Nummulites and Orbitoides ; 

 which, radiating upwards and outwards from this eminence, 

 passes into the general structure of the shell. 



The transitionary forms of the chambers in Operculina, through 

 Nummulites and Orbitoides to Orbitolites, would, when viewed in 

 a vertical section, appear to be thus : — In Operculina there is a 

 single plane of spear-head shaped chambers ; in Nummulites a 

 central plane of conical chambers with layers of compressed ones 

 above and below it ; in Orbitoides, a central plane of quadran- 

 gular chambers with numerous layers of compressed ones above 

 and below it ; and in Orbitolites, a mass of circular or ovoidal 

 cells more or less definitely arranged. Hence, if these be their 

 respective peculiarities, Orbitoides Mantelli will, from Dr. Car- 

 penter^s illustration (fig. 31, loc. cit.), belong to the latter, and 

 would therefore be now more properly named Orbitolites Man- 

 telli. 



One other observation I would here make with reference to 

 geology, viz. the natural union which now seems to be pointed 

 out between the Chalk and the Nammulitic series, by the great 

 prevalence of the same class of animal remains in each — that is 

 to say, the abundance of flints which indicate the previous ex- 

 istence of siliceous sponges in the former, and the myriads of 

 Foraminifera which are closely allied to the calci-sponges in the 

 latter. 



Bombay, May 12, 1852. 



EXPLANATION OF PLATE IV. 



Fig. \. Operculina Arabica, natural size. 



Fig. 2. Large and small papillae on the external surface of the shell, highly 

 magnified. 



Fig. 3. Vertical section of the shell over the chambers, highly magnified, 

 showing : — a, a, large papillae ; b, b, small ditto ; c, c, horizontal 

 lines indicative of the layers of the shell ; d, d, vertical tubes. 



Fig. 4. Spicula in situ, highly magnified. 



Fig. 5. Diagram of horizontal section of three large chambers of the shell, 

 showing the interseptal vascular system and spicular cord : a, a, a, 

 chambers ; b, b, b, septa ; c, c, c, interseptal vessels ; d, d, d, ra- 

 musculi coming to the surface of the shell ; e, e, e, e, ditto, going 

 to the walls of the shell, &c. through the septa, the dotted lines 

 indicating those branching out into the former; /,/,/, marginal 

 plexus ; g, g, g, ramusculi of margin ; h, h, spicular cord ; i, half- 

 formed septum with termination of interseptal vessel. 



Fig. 6. Diagram of vertical section of the shell to show the form of the spi- 

 cular cord : a, margin or free surface of spicular cord ; b, vertical 



