260 Messrs. R. B. Newton and R. Holland on some 



does not lend itself to this supposition. Tho general arranf^e- 

 ment of the cliamberlets is shown in fig. 3 c, which is drawn 

 from a horizontal section near, but not at, the median plane. 

 A transverse section shows the median disc, which does not 

 appear to be quite uniformly central in its position, exceed- 

 ingly thin in the middle, thickening rapidly towards the 

 circumference, rounded at the margin, and having somewhat 

 the contour in section of an hourglass drawn out a little at 

 the ends. The primordial chamber, as far as can be made 

 out, is very small." 



In our PI. X. figs. 7 and 8 we figure again the specimens 

 illustrated by Brady in the * Geological Magazine.' Brady's 

 figures were drawn by A. HoUick, ours are from photographs 

 of the actual specimens, and simply demonstrate the accuracy 

 of the original drawings. In fig. 10 we give Brady's prepara- 

 tion (not before figured) on which he appears to have based 

 his description of the transverse section of the shell. It is 

 clear that this figure does not represent a vertical section, but 

 one taken at a considerable angle with the vertical. 



An example of 0. sumatrensts in the British Museum 

 (Nat. Hist.) collection has been ground down in order to 

 discover the true characters of the chambers of the median 

 plane (PI. X. fig. 9). In thus operating upon this specimen 

 we were able to note the appearance of the horizontal sections 

 at various stages and also sections at slight angles with the 

 horizontal. We have thus been enabled to recognize that 

 numerous orbitoid sections in our slides cut from the lime- 

 stone of Gomanton Hill and from one of the pebbles from 

 the bed of the River Malinam are to be properly referred to 

 this species ; and we venture to give the characters of the 

 species as follows : — 



Characters. Shell subglobular ; about 3 millim. in width 

 by 1*5 to 2*5 millim. in thickness; exterior rough and 

 granular ; median edge produced to form a narrow keel ; 

 median chambers variable in size and shape, but always 

 rounded or '' spatuliform." Chambers above and below the 

 median plane have the shape of shallow cylinders and are 

 remarkably constant in size and regular in arrangement ; 

 chamber-walls finely perforate. The initial chamber of the 

 British Museum specimen was " megalospheric " (about 

 •5 millim. in diameter) ; the Cambridge specimen (PI. X. 

 fig. 10) is possibly, as Brady supposed, " microspheric." 

 If so, we have botii form A and form B of the species. 



The general external a])pearance of the shell is shown in 

 PI. X. fig. 7, photographed from Brady's figured specimen. 



