18 THE VOYAGE OF H.M.S. CHALLENGER. 



were sulodivided by partitions answering to its surface-markings (fig. 7, p. 46), spring- 

 ing from their inner septa between the j^ores, but not extending to their outer, the result 

 would be exactly what we here find. 



Now the peculiar point of interest attaching to this Orhitolites tenuissima, is that 

 the structure of the inner part of every disk shows it to have thus originated : for the 

 " nucleus " is here a continuous spii-e of five or six turns (PI. II. fig. 3), closely 

 resembling that of a young Cornuspira, with an indication of imperfect septal 

 interruptions resembling those of a Sjnrolocvlina ; the spire, when beginning to open-out 

 (fig. 5, a), is interrupted by a complete septum traversed by pores, exactly corresponding 

 to that of PeneropUs ; whilst the next chamber, b, is divided into four chamberlets by 

 three partitions springing from this septum between its pores, this subdivision converting 

 the incipient PeneropUs into a young Orbiculina. In the specimen here figured, this 

 chamber is not separated by a completely-formed septum from the next chamber c, and 

 the latter is undivided save by a single radial partition ; and although this is a mere 

 individual variation, it is of interest as showing a reversion to the " peueropline " type, 

 even after the assumption of the " orbiculine." The orbiculine type prevails through 

 several succeeding chamber-additions ; but the spiral plan of gTowth characteristic of it 

 soon begins to give place to the cyclical ; for the next-formed chamber cl, d, which is 

 divided into chamberlets by radiating partitions that spring from the inner septum 

 between the pores, sends backwards alar extensions d', d', which begin to enclose the 

 spiral " nucleus." This extension is still more marked in the next chamber e, e, whose 

 two alte, e', e', reach the ends of the transverse diameter of the original spire ; and the 

 alse of the subsequently-formed chambers extend themselves further and further back 

 around the spire (as shown in PI. I. fig. 1), until — in the specimen here figured — those 

 of the ninth chamber meet at the opposite side of the spire, so as to enclose it aU round, 

 while the tenth forms the first complete annulus, to be itself suiTounded by a succession 

 of similar annuU, the number of which in full-sized specimens may exceed thirty. 



Thus we have, in this one organism, a complete transition from the simple slightly 

 interrupted spiral tube of the least diff'erentiated Miliolines, through the expanded and 

 chambered spire of PeneropUs, and the chamberletted spire of OrhicuUna, to the con- 

 centric annulation and subdivided chambers of the typical Orhitolites. And we shall 

 presently see how this last plan undergoes, in other species, a progressive modification, 

 until, in its most specialised types, we lose all trace of derivation from a spu'al, — the 

 anuuli being formed concentrically, from their very commencement, around a discoidal 

 " nucleus," and their chamberlets being so modified in shape and disposition, as not to 

 suggest their origin in the subdivision of a Pe/ierop?is-chamber by radial partitions. 



Rejyarations. — As might be expected from the extreme tenuity and fragility of the 

 disks of this species, they are obviously very Liable to fracture ; scarcely any specimen 



I 



