DE. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 553 



be derived by a further division of the segments of the sarcodic body into sub- 

 segments, with a corresponding division of the primary chambers of the shell into 

 chamberlets. 



In the new specific type of Orbitolites I have now to describe, the whole transition 

 which I thus hypothetical ly indicated, is actually presented during the successive 

 stages of its growth. For it begins life as a Cornuspira, taking-on that ' spiroloculine ' 

 condition which marks the passage towards the Milioline type : its shell forming a 

 continuous spiral tube, with sh'ght interruptions at the points at which its successive 

 extensions commence ; while its sarcodic body consists of a continuous coil, with 

 slight constrictions at intervals. The second stage consists in the opening-out of its 

 spire, and in the division of its cavity at regular intervals by transverse septa, 

 traversed by separate pores, exactly as in Peneroplis. The third stage is marked by 

 the subdivision of the ' peneropline ' chambers into chamberlets, as in the early forms of 

 Orbiculina. And the fourth consists in the exchange of the spiral for the cyclical 

 plan of growth, which is characteristic of Orbitolites ; a circular disk of progressively 

 increasing diameter being formed by the addition of successive annular zones around 

 the entire periphery. This increase in diameter is not here accompanied (as it is in 

 most of the other forms of the Orbitoline type) by a corresponding augmentation in 

 thickness ; and as the extraordinary tenuity of these disks affords an easily recognisable 

 and (as I believe) a constant differential character of the species, I proposed in 1870* 

 to designate it Orbitolites tenuissima. 



ORBITOLITES TENUISSIMA. Carpenter, 1870. 



The disks of 0. tenuissima are usually almost perfectly flat (Plate 37, fig. 1), and 

 exhibit a remarkable regularity of structure. The diameter of the largest complete 

 specimen I have seen is not above 0'25 inch ; but it is obvious from the size and 

 curvature of the fragments which the dredges frequently contained, that they must 

 have belonged to disks whose diameter was at least 0'6 inch, these larger specimens 

 having come to pieces in their rough removal from the soft and tranquil ooze on which 

 they had previously lain. This fragility depends in part upon the extreme tenuity of 

 the disks, their thickness rarely exceeding one three-hundredth of an inch ; and in part 

 on the slightness of the connexion which (as I shall presently show) exists between 

 the successive zones.t 



When either surface of the disk of 0. tenuissima is viewed by reflected light 

 under a low magnifying power, its concentric zones are seen to be crossed by radial 

 lines (Plate 37, figs. 1, 2) resembling those which pass between the septal bands 



* Proc. Roy. Soc., vol. 19, p. 176. 



f In Cyclodypeus, the marginal portions of the disk, though of even greater tenuity, have not by 

 any means the same fragility ; partly because its vitreous shell-substance is much firmer than the 

 porcellanous shell-substance of Orbitolites, and partly because a layer of it is usually continued from each 

 new zone over the whole surface of the previously formed disk. See Pbil. Trans., 1856, p. 558. 



