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. L, 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 adunca (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 O. 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-shaped " primordial 

 chamber," from the neck of which proceeds a " circumambient chamber " that passes 



Fig. I. Orliitolites 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. 



