452 J^'ATURE AND 31 AN. 



aiHfia, coiling continuously round a primordial chamber, as in 

 Cornuspira. Thus, in this interesting organism we find permanently 

 represented the whole developmental history of the " simple " type 

 of Orbitolite from the primordial jelly-speck. The large Challenger 

 collection of Orbitolites, made on the Fiji reef, has furnished me 

 with the means of still more completely working out the transition 

 from the "simple" to the "complex" type; a distinctly inter- 

 mediate type there presenting itself in great abundance. This, 

 which I term the "duplex" type (Fig. VL, i), resembles the 

 "simple" in having its annular series of chamberlets disposed in 

 a single plane, and in the connection of the chamberlets of each 

 ring by a single annular canal ; but differs in having its successive 

 rings connected by a double series of radial passages, which issue 

 on the edge of the disk (Fig. VI., 2) as marginal pores. The 

 columnar sub-segments, a a', b b\ of each ring are strung, as it 

 were, on the annular cord, c c' ; and this sends off an upper and 

 a lower series of stolon-processes, d d, d'd', which pass into the 

 upper and lower halves of the sub-segments of the next ring. — 

 The plan of growth in the first-formed portion, shown in Fig. VL, 

 3, is singularly intermediate between that of the "simple" and that 

 of the " complex " type. The regular spire of the former is now 

 reduced to the single turn made by the " circumambient segment," 

 b b, round the "primordial segment" a ; but a partial continuance 

 of the same plan is shown in the incompleteness of the first two or 

 three rings of sub-segments ; these being budded forth from only 

 half of the " circumambient segment," instead of from its whole 

 periphery, as in the typical "complex" Orbitolite (¥\g. II.). Yet 

 even in large disks, whose later growth is characteristically " com- 

 plex," the nucleus and earlier rings are often formed on the 

 " duplex " plan, which passes into the " complex " in the manner 

 to be now described. 



Believing, with Sir James Paget, that " the highest laws of 

 "biological science are expressed in their simplest terms in the 

 " lives of the lowest orders of creation," I shall now ask you to 

 follow me through a detailed examination of the transition from 

 one type to the other ; as shown in Fig. VII., which represents 

 a vertical section, taken in a radial direction, of one of those large 

 "complex" disks whose life was commenced on the plan of the 



