94 BULLETIN" 71, UNITED STATES NATIONAL MUSEUM. 



Up to and including chamber 10 the test is close coiled, although 

 increasing rapidly in height, but with chamber 11a new character is 

 initiated, that of bending back on one side as was seen in chamber 

 21 in Orbiculina compressa. 



Chamber 20 in this specimen of Orbilolites marginalis completes 

 the circle of the periphery of the test and forms the first annulus. 

 This corresponds with chamber 35 in Orbiculina compressa, where 15 

 chambers are necessary to complete the stage from the first bending 

 back to the formation of the first annulus, which is in Orbitolites 

 marginalis accomplished by the addition of but 10 chambers. This 

 illustrates well the difference in the acceleration of development of 

 these two specimens of the two different species. 



ORBITOLITES DUPLEX Carpenter. 

 Plate 38, figs. 3,4; plate 39, fig. 1. 



Orbulites macropora (?) Lamarck, Hist. Nat. Anim. sans Vert., vol. 2, 1816, p. 197, 



No. 5 (fide Carpenter). 

 "Orbitolites (duplex type)" Carpenter, Philos. Trans., 1856, p. 120, pi. 5, fig. 10; 



pi. 9, fig. 10. 

 Orbitolites complanata, var. macropora Parker and Jones, Ann. Mag. Nat. Hist., 



ser. 3, vol. 5, 1860, p. 289. 

 Orbitolites duplex Carpenter, Rep. Voy. Challenger, Zoology, pt. 21, 1883, p. 25, 



pi. 3, figs. 8-14; pi. 4, figs. 6-10; pi. 5, figs. 1-10.— H. B. Brady, Rep. Voy. 



Challenger, Zoology, vol. 9, 1884, p. 216, pi. 16, fig. 7, fig. 7, in text. 



Description. — Test in the form of a circular disk, the surface flat 

 or slightly concave toward the center, chambers numerous, soon 

 becoming annular; wall smooth, chambers and cham- 

 berlets obscurely marked, apertures in the adult con- 

 sisting of a double row of openings on the periphery 

 near the margin, corresponding to the double tier of 

 chamberlets, usually alternating. 

 Diameter, up to 8 mm., but usually much less. 

 Distribution. — Apparently the only definite North 

 Pacific record for this species is that of Rhumbler, who 

 Fl cHAMBm!s S oF o^ records the species from Laysan Island. Near the 

 bitolites duplex, Hawaiian Islands it has occurred at Nero station 2033 

 x m in 249 fathoms and 2071 in 271 fathoms. It also 



occurred in some numbers at Nero station 1466 in 234 fathoms off 

 Guam. 



The first four chambers of a specimen of Orbitolites duplex with 

 a megalospheric proloculum are shown in figure 48. As in O. mar- 

 ginalis there is an oval proloculum followed by a long Oornuspira- 

 like chamber of nearly a coil in length of equal thickness with the pro- 

 loculum and building no floor. Chamber 3 is simple with two apertures 

 as in O. marginalis. Chamber 4 is divided into two chamberlets. 



