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



rather thin coin. Departures from the typical flatness, however, are not unfrequent ; 

 one of the most common being a sort of plaiting into radial folds, which, beginning near 

 the centre, increases towards the margin — as is slightly indicated in the above-cited 

 figure, but is more strongly shown in the specimen represented in PI. XVI. fig. 7 of 

 Mr. Brady's Report on the Foraminifera. Each surface of the disk is marked-out into 

 regular concentric annuli ; but the division between these is not so conspicuous as in 

 Orbitolites margincdis ;' and specimens not unfrequently present themselves, whose surface 

 in certain aspects looks rather " engine-turned " (like that of the section shown in PL IV. 

 fig. 6) than concentrically annular. This appearance, however, does not mark any differ- 

 ence of internal structure, and seems to depend upon the manner in which the light is 

 reflected from the thin films of shell-substance that cover-in the individual chamberlets, 

 which are often slightly convex. When a portion of the surftice of a specimen con- 

 taining the sarcodic body of the animal is viewed under a sufficient power by reflected 

 light (PL III. fig. 10) the cavity of every chaml:>erlet is marked by a circular or oval spot, 

 surrounded by a thick wall of shell-substance, which is divided by a definite line from the 

 walls of contiguous chambers. It is further noticeable that the double wall which thus 

 separates two contiguous chamberlets of the same annulus, is quite as thick as that which 

 separates the chamberlets of consecutive annuli. The chamberlets of successive annuli 

 generally alternate with one another in position, so as to lie in oblique row^, which, 

 when the interior of the disk is viewed under a low magnifying powder (PL IV. fig. 6), 

 seem like parts of excentric cu-cles. 



The inner (first-formed) portion of the disk in Orbitolites duplex shows only a very 

 slight approach to that " orbiculine " spire which is typical alike of Orbitolites tenuissima 

 and of Orbitolites marginalis, approximating much more closely to the true cyclical plan 

 of Orbitolites compktnata. The nucleus consists, as in Orbitolites mai'ginalis, of a 

 smaU primordial chamber, which is surrounded by a circumambient chamber ; and 

 round this nucleus is seen (PL III. fig. 14) a row of chamberlets, which often at 

 once forms a nearly entire annulus, the ring being soon completed in succeeding circlets, 

 and all subsequent additions being made on the cyclical plan. When this nucleus is 

 examined in thin section (PL IV. fig. 10) it is seen that this early assumption of the 

 cyclical plan arises from the fact that the circumambient chamber b, b' gives ofi" several 

 passages on its outer margin, which lead into as many chamberlets ; so that it is as 

 completely surrounded by chamberlets, after thi-ee or four successive additions, as it is in 

 Orbitolites marginalis (fig. 4) after twelve or more. In this particular, then, Orbitolites 

 duplex presents us with a very interesting transition from Orbitolites marginalis, in which 

 only a single chamber is put forth from the extremity of the circumambient chamber, 

 to Orbitolites complanata, in which it sends forth passages round its entire margin, 

 so that the very first series of chamberlets forms a complete annulus (PL VI. figs. 1, 2, 3). 

 The arrangement of parts in the sarcodic body of the animal (PL V. fig. 6) entirely 



