DR. 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 hypothetically indicated, is actually presented during the successive 
stages of its growth. For it begins life as a Cornuspira, taking -011 that £ spiroloculine ’ 
condition which marks the passage towards the Milioline type : its shell forming a 
continuous spiral tube, with slight 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. 
t In Cycloclypeus, 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. 
