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



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

 cells in the centre of one of these papyraceous Orbitoliies (Ct/clo- 

 lina, 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 ; 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 incrnsted surface of Cyclolina 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 1 -633rd of an 

 inch in diameter. 



EXPLANATION OF PLATE III. 13. 



Fig. 1. Melonites sphceroidea (Lam.), magnified four diameters: a, ribs; 

 b, segments. 



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

 men or umbilical hole. 



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

 proportionally magnified : a, 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 

 the rib ; c, free space or channel behind and internal to the rib ; 

 d, d, d, chambers ; e, e, septa. 



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

 angular canal of rib ; b, free space or channel behind and internal 

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



Bombay. 14th April, 1854. 



