216 
THE YOYAGE OF H.M.S. CHALLENGER. 
Orbitolites duplex, Carpenter [macropora, Ehrenberg, sp.?] (PL XYl. fig. 7). 
Orbulites macropora (1), Lamarck, 1816, Hist. Nat. Anim. s. Yert., vol. ii. p. 197, No. 5 (Jide 
Carpenter). 
Orbitolites macropora (1), Goldfuss, 1826, Petrefacta Germanise., &c., vol. i. p. 41, pi. xii. fig. 8, a. b. 
Amphisorus liemprichii, Ehrenberg, 1839, Abhandl. d. k. Akad. Wiss. Berlin (for 1838), p. 134, 
pi. iii. fig. 3. 
“ Orbitolites, duplex type,” Carpenter, 1856, Phil. Trans., p. 120, pi. v. fig. 10; pi. ix. fig. 10. 
Orbitolites complanata, var. macropora, Parker and Jones, 1860, Ann. and Mag. Nat. Hist., 
ser. 3, vol. v. p. 289 ; — vol. vi. p. 38, No. 35. 
,, duplex, Carpenter, 1883, Report on the Genus Orbitolites, Zool. Chall. Exp., part 
xxi. p. 25, pi. iii. figs. 8-14 ; pi. iv. figs. 6-10 ; pi. v. figs. 1-13. 
The forms distinguished by Dr. Carpenter, in his recent Challenger Report, under the 
name Orbitolites duplex, though referred to in his earlier accounts of the genus, had not 
previously 1 sen separated in a specific sense from Orbitolites complanata. They never- 
theless appear to exemplify well differentiated and tolerably constant structural 
peculiarities, equally removed from those of Orbitolites marginalis on the one hand, and 
Orbitolites complanata on the other. They represent in fact an intermediate stage 
somewhat more complex than the former, but not attaining the full development of the 
latter type. 
The shell of Orbitolites duplex, like those of its congeners, takes the form of a 
circular disk, with flat or very slightly concave surfaces. The diameter never exceeds 
about ^rd inch (8'4 mm.) and the maximum thickness, at or near the margin, about -^th 
inch (0'3 mm.). The central portion or nucleus presents intermediate characters. It 
consists of a primordial chamber surrounded by a circumambient chamber, and this is 
followed by two or three incomplete circlets, after which the cyclical mode of growth 
commences immediately without the interposition of any distinct series of spiral seg- 
ments. Both the septal walls and the secondary septa are thicker, in proportion to the 
size of the cavities they enclose, than those of Orbitolites marginalis, and the concentric 
annuli are in consequence less distinctly marked on the exterior. On the lateral faces of 
the test the chamberlets appear in the shape of circular spots, each bounded by its own wall ? 
and those of the adjacent annuli alternate more or less regularly with each other. The 
peripheral face is marked by a double row of pores, which serve collectively as the 
aperture of the test. The pores of the two rows are generally but not invariably placed 
alternately. 
The double row of external orifices marks the chief distinctive feature of the test, and 
indicates the point in which the internal structure differs from that of the simple type. 
The disk of Orbitolites marginalis, as already stated, consists of a single tier of chamberlets, 
each circlet of which has its own annular gallery into which all the constituent chamber- 
lets open freely, and from which a single series of radial passages are given off connecting 
it with the succeeding annulus. The test of Orbitolites duplex, on the other hand, is 
