Mr. H. J. Carter on the Structure of the Shell of Alveolina. 101 



I have also lately been so fortunate as to obtain a view of the 

 cells in the centre of one of these papyraceous Orbitolites [Cyclo- 

 Una J D^Orbig.) — the doubly concave species — and I find that 

 they are arranged in concentric circles there, though apparently 

 not more than one deep. The greatest care therefore is neces« 

 sary in exposing them, for a rub too much or too little may 

 either destroy them altogether or entirely fail to bring them 

 into view. I mention this, because I had inferred, from the sub- 

 spiral arrangement of the cells in Orbitoides Prattii and Orbito- 

 lites Malabarica (Plates VII. & XVI. vol. xi.), that this might be 

 the case throughout D^Orbigny's order Cyclostegues j but on 

 actual examination and subsequent reflection, I find that such 

 cannot be the case in Cyclolina ; at the same time, the arrange- 

 ment of the cells being just like that of the divisions on an 

 engine-turned watch-case, to which Dr. Carpenter has so aptly 

 likened them in Orbitolites complanata, Lam. (Quart. Journ. Geol. 

 Soc. vol. vi. pt. 1. p. 24), I think this way of describing them 

 much more intelligible than by the use of the term " concentric." 

 The circles on the incrusted surface of Oyclolina are evidently 

 concentric ; they are in continuous lines ; but the arrangement of 

 the cells beneath the crust, though still seen to be concentric 

 under a high microscopic power, fails at first to strike the eye as 

 such from the greater prominence of the semi-gyrating lines, 

 which, flying off" from the centre, most faithfully represent the 

 appearance mentioned. 



I have measured the cells in the specimen in which I observed 

 this arrangement most satisfactorily, and find that those of the 

 centre are much smaller than those of the circumference ; the 

 former are l-380th of an inch, and the latter l-633rd of an 

 inch in diameter. ^ ...H^sO^-^ gooo') 



EXPLANATION OF PLATE IIL b! in = 



Fig. 1. Melonites sphcsroidea (Lam.), magnified four diameters: a, ribfl|>s^ 



b, segments. ' -..., 



Fig. 2. Extremity of the same, magnified four diameters : a, central fora- - 



men or umbilical hole. 

 Fig. 3. Longitudinal section of the same through the centre, greatly but 

 proportionally magnified : o, a, chambers ; b, canals and parti- 

 tions in the wall of the shell. 

 Fig. 4. Transverse section of the same through the centre, equally mag- 

 nified, &c. : a, spiral wall of the shell ; b, triangular canal of 

 Ji^- 'V the rib ; c, free space or channel behind and internal to the rib ; 

 •)?'^ii^ rf, c?, c/, chambers ; e, e, septa. 

 f^g. 5. Magnified view of a portion of the wall of the shell, to show — a, tri- 

 angular canal of rib ; b, free sj>aoe or channel behind and internal 

 "^ to it ; c, c, partitions of canaliferous structure. 



Bombay, 14th April, 1854. 



