W. B. CARPENTER ON THE STRUCTURE OF ORBITOLITES. 99 



next stage corresponds with the Milioline Spiroloculina, the spire 

 being constricted at intervals by imperfect partitions ; and this passes 

 into the Peneropline stage, in which the partitions are numerous 

 and complete (Fig. 7). The next stage is that of the Orbiculina, 

 found in tropical seas so abundantly that nearly every handful of 

 sand contains them ; in which the principal chambers are divided 

 into chamberlets, and the spiral plan of growth gives place to the 

 cyclical. This brings us to the Orbitolite itself ; and of my previous 

 strong impression that it was developed from the simpler Milio- 

 lines, I had a complete confirmation in the beautiful deep sea form 

 0. tenuissima. 



When the Challenger expedition brought home its results, Sir 

 Wyville Thomson placed in my hands a jar of Orbitolites from the 

 reefs of Fiji ; and I undertook to work out this collection, thinking 

 that so large a gathering from one locality might enable me to 

 throw some more light on a good many questions of development. . 

 . . (The specimens exhibited upon the table form a complete series 

 in illustration of Orbitolite structure ; the largest of them showing 

 a remarkable exuberance, in the shape of a number of curious out- 

 growths.) I found that this collection, when sorted out, fell 

 naturally into very distinct groups. First there was a remarkable 

 series of specimens as simple in their plan of structure as the 

 Mediterranean 0. marginalis, but very much larger (Fig. 8) ; then 

 of the duplex Hed Sea form (Fig. 9) ; and then of the large 0. 

 complanata (Figs. 4, 10). The question of species then came up 

 again ; and with this large collection I saw at once that each of 

 these types had its own size, plan of growth, and general aspect, so 

 that it was very easy to distinguish between them ; and yet a most 

 complex form might show that it had passed through the grades 

 of the simple and the duplex (Fig. 11). Both these show a 

 survival, in their early growth, of the original spiral plan (Fig. 

 7, 5,6 ) ; but in the most typical specimens of the large 0. compla- 

 nata, the plan is circular from the very commencement (Fig. 7, 7 ). 

 Another point of interest was that the fossil forms stopped in an 

 incomplete stage ; for it was easy to see in vertical sections of fossil 

 forms, that the continuity of the superficial and the intermediate 

 chamberlets was maintained throughout (Fig. 11, e, e) ; while in the 

 large recent type they are disconnected by a shifting to half the 

 breadth of a ring (f,f\ f 2 f 3 ). In recent specimens from other 

 localities I have found the earlier condition shown in the fossil 



