1883.] 



Researches on the Foraminifera. 



277 



the shell, with a partial interruption of the tube at these points, gives 

 us Spiroloculina, the ancestral form of the whole Milioline series ; 

 (3) that the flattening-out of this tube (as in the more advanced 

 growth of Cornuspira), and the formation of a complete septum across 

 its mouth, traversed by separate pores for the issue of sarcodic fila- 

 ments, convert it into a Peneroplis, whose sarcodic body consists of a 

 succession of segments, occupying the successive chambers of the 

 shell, which are divided by septal partitions, but are connected by 

 " stolon processes " that traverse them ; (4) that the subdivision of 

 these chambers by transverse partitions into " chamberlets," and of 

 the segments of the body into " sub-segments," the shell still growing 

 along a spiral axis, gives us Orbiculina ; the chamberlets of each 

 chamber retaining a communication with each other by a continuous 

 gallery, which is occupied in the living state by a band of sarcode that 

 connects together all the sub-segments formed by the division of any 

 one segment ; and (5) that -the opening-out of the Orbiculine spire, 

 and the progressive extension of the alee, of its successively-formed 

 chambers round its umbilicus, at last brings these alee together, so 

 that they unite into a continuous ring ; and by the addition of new 

 rings to the periphery of the preceding, a circular disk is formed — the 

 plan of growth thus changing from the spiral to the cyclical, which is 

 the distinguishing character of Orbitolites. 



The materials at that time possessed by the author only enabled him 

 to trace back this pedigree with any certainty from the typical Orbito- 

 lites, which exhibits no trace whatever of spiral growth, to the spiral 

 Orbiculina ; but he expressed his belief that the " nuclear mass " in 

 which every Orbitolites originates — consisting of a pyriform " primor- 

 dial segment," surrounded by a single turn of the " circumambient 

 segment," is essentially an abbreviated Milioline; and he was thus led 

 to rank Orbitolites as the most specialized type of the family Miliolida. 



It was, therefore, with singular satisfaction that he found in a new 

 form of Orbitoline disk, brought up from a depth of 1,500 fathoms in 

 the "Porcupine " expedition of 1869, the complete realisation of his 

 hypothetical pedigree: the formation of this disk commencing in a 

 minute primordial chamber, which first extends itself into a closely 

 coiled spiral tube like that of a Cornuspira, then shows an incipient 

 septation in the later coils of this tube, which constitutes it a Spiro- 

 loculina ; then flattens-out and becomes camerated as a Peneroplis ; then 

 undergoes the subdivision of its chambers which converts it into an 

 Orbiculina ; and, finally, by the fusion of the lateral extensions of the 

 chambers into complete annuli, assumes the cyclical plan of growth 

 characteristic of Orbitolites. To this beautiful species, whose disk 

 attains a diameter of ~ of an inch, whilst its thickness does not 

 exceed of an inch, the name Orbitolites tenuissima may be appro- 

 priately given. 



