560 
DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
which I formerly* differentiated only by the singleness of its row of marginal pores 
(fig. I., 3), I now find to be uniformly characterised by the marked eccentricity of their 
primordial chamber, and by the spiral direction of their early growth (fig. I., 2), which 
I formerly supposed to be only occasional variations. In fact, the first formed portion 
of these disks, like that of a young Orbiculina aclunca (loc. cit., Plate XXVIII., fig. 2), 
exactly resembles what a Peneropline shell would be, if its chambers, as they 
widen out, were to undergo division into chamberlets ; thus corresponding in every 
essential particular with the “ orbiculine ” stage of 0. tenuissima. But while we have 
seen that this stage, in the last-named species, is preceded by a spiroloculine coil, 
representing a true “ milioline ” stage, it has no other predecessor in 0. marginalis than 
what I formerly designated as the “nucleus,” consisting of a flask-shapecl “primordial 
chamber,” from the neck of which proceeds a “circumambient chamber” that passes 
Fig. I.— Orbitolites marginalis. 
almost completely round it (as in fig. II., 3). The morphological import of this 
arrangement becomes clearer when we compare the sarcodic bodies of the two types; 
for it is then obvious that the “circumambient segment,” which springs from the 
“ primordial segment,” and then, after making a single coil around it, gives off the first 
“peneropline ” segment, really represents the multiple spiroloculine coil of 0. tenuis¬ 
sima ; this early generalised “milioline” stage being (as it were) abbreviated with the 
advance towards specialisation, as we see in numberless cases elsewhere. 
The “ Challenger” collection—-especially that of the 18 fathoms’ dredging—includes 
a very large number of thin flat disks, attaining a diameter of about 0 - 32 inch, whose 
surface often presents rather an “ engine-turned ” than an annular aspect, and which 
are specially characterised by the possession of a complete double row of marginal 
Phil. Trans., 1856, p. 215, Plate VII., fig. 14. 
