REPORT ON THE GENUS ORBITOLITES. 7 



the transition is almost insensible ; and from the Spiroloculine we pass by easy steps to- 

 all the other forms of the Milioline types." Again, a subdivision of the widely- expanded 

 spire of Cornuspira into segmental chambers, gives us PeneropUs, with its septal planes 

 perforated by a row of separate pores ; while from this, it was again pointed out, the 

 spiral Orbiculina might be derived by a further division of the sarcodic body into sub- 

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

 chamberlets. 



It was therefore with no small satisfaction that I recognised, among the products of 

 the deep-sea dredgings carried on in the " Porcupine " expedition of 1869, a " missing 

 link " that reproduces the whole of this genetic series in its own single organism, namely, 

 a chambered calcareous disk, of which, though nearly the whole is constructed on the 

 typically Orbitoline plan, the central (or youngest) part shows, in the first place, the 

 simple undivided tubular coil of a young Cornuspira ; then the partial interruption of 

 that coil by incomplete septa, as in Spiroloculina ; then the fiatteuing-out of the spire, and 

 its partitioning into chambers by perforated septa, as in Penerop>lis ; then the subdivision 

 of the spirally-growing chambers into chamberlets, as in Orbiculina ; and finally, the 

 substitution of the cyclical for the spiral plan of growth, constituting it a true OrbitoUtes, 

 — as will be presently set forth in detail in the description of OrbitoUtes tenuissima. 



I had pointed out (Phil. Trans., 1860, p. 574) that the .shells of the whole of 

 this series — -together with that of the fusiform Alveolina, which I regarded as 

 another derivative from the same fundamental type — have that p>orcellanous character, 

 whose distinctive importance was first indicated by Prof W. C. Williamson, though he 

 did not venture to adopt it as a basis of the primary subdivision of the group ; and that 

 a precisely parallel relation exists among those generic types of the series forming vitreous 

 shells, which present the most highly specialised forms of Foraminiferal organisation. 

 For whilst Opercidina is (so to speak) a " vitreous " PeneropUs, and Heterostegina a 

 " vitreous " Orbiculina, we have in Cycloclypeus, which shows a perfectly cyclical 

 mode of growth in a finely tubulated shell, the " vitreous " parallel of OrbitoUtes ; the 

 parallelism being completed by the existence, in the probably "vitreous" FusuUna,^ 

 of the same plan of growth around an elongated axis as is shown in the " porccllanous " 

 Alveolina. 



In the same concluding summary (1860) I presented, as results of my researches, 

 certain "general propositions" (p. 584), which I think it desirable here to reproduce; 

 because, as my original investigation of the forms of the genus OrbitoUtes then known to 



' I was obliged at that time to speak with hesitation of the place thus assigned to Fusulina (whose fossil shells 

 make up the bulk of certain beds of Carboniferous limestone in Russia and elsewhere), "the metamorphic condition of 

 its shell interfering with the minute study of its structure"; but a subsequent examination of specimens well preserved 

 in the clays of the Carboniferous limestone of Iowa has satisfied me that my original interpretation of its microscopic 

 appearances was correct {Monthly Mia: Journ., vol. iii. 1870, p. 18(i). By previous systematists, Fusulina had been 

 generally associated witli Alveolina, to whicli its external resemblance is most remarkable. 



