554 DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 



of Peneroplis. But when a portion of the disk is viewed under a higher power 

 by transmitted light, -which, through the extreme tenuity of its superficial lamellae, 

 brings its internal structure into distinct view (Plate 38, fig. 5), these lines are seen 

 not to be mere surface-markings, as in Peneroplis, but to be the indications of internal 

 shelly partitions, which divide each flattened annular chamber into a series of narrow 

 chamberlets, resembling those which I formerly described as constituting the two 

 superficial layers of the "complex" type of Orbitolites.* Here, however, these 

 chamberlets form but a single plane, as in the " simple " type formerly described ; 

 and the pores by which the last-formed annulus opens at the margin of the disk are 

 arranged in single series (Plate 37, figs. 4, 5). It is worthy of note that these 

 pores are not round, like those of ordinary Oi'bitolites, whether of the " simple " or of 

 the " complex " type ;t but are more or less elongated in the plane of the disk 

 a peculiarity obviously related to its extreme compression. Similar pores are 

 seen upon the edge of any zone from which the zone external to it has been 

 detached by fracture ; and it is obvious that they constitute the channels of commu- 

 nication between the cavitary system of each zone and that of the zones internal and 

 external to it ; while the marginal series brings the cavitary system of the peripheral 

 zone (and, through it, that of every interior zone backwards to the spiroloculine 

 " nucleus ") into relation with the surrounding medium. 



When a portion of the thin shelly lamella forming either surface of the disk has 

 been removed by dilute acid, so as to lay open the cavity beneath (Plate 37, fig. 2), 

 it is seen that each zone of chamberlets lies between two concentric rings of shell, 

 a, a, b, 6 ; and that the radiating partitions, c, while springing from the inner shell- 

 ring, do not extend to the outer, so that a continuous gallery is there left, into which 

 all the chamberlets open at their peripheral extremities. And when we examine the 

 disk by transmitted light (Plate 38, fig. 5), we see it to be from this gallery not 

 from the chamberlets that the pores of the shell- ring which incloses it proceed. 



Whilst the structure of the concentric zones forming the peripheral portion of the 

 disk thus corresponds in all its essential characters with that of the ordinary 

 " simple " type described in my former Memoir, the structure of the central portion of 

 the disk is altogether different. The spheroidal " primordial chamber " (Plate 38, 

 fig. 3, a) is extremely minute, not exceeding 1-1 000th inch in diameter, and from 

 this proceeds a compressed shelly tube, which forms a nautiloid spiral around it 

 (Plate 38, figs. 3, 5) each successive turn slightly increasing in breadth, so as closely 

 to resemble the first-formed part of the spire of Cornuspira. The continuity of its 

 cavity, however, is interrupted, usually at about every two-thirds of a turn, by a 

 thickening of its wall (Plate 38, fig. 3, b), which seems to have been formed as a sort 

 of foreshadowing of a septum at each addition to its length ; and thus, as long as the 

 growth of the shell proceeds upon the same plan, it is a ' spiroloculine ' Miliola. 



* Phil. Trans., 1856, p. 202, Plate V., fig. 6, c, c; Plate VII., fig. 12. 

 t Ibid., Plate V., figs. 1 and 6, d, d. 



