642 
THE YOYAGE OF H.M.S. CHALLENGER. 
The shell-wall is hyaline, and often, especially amongst the larger forms, coarsely 
perforated ; but this is by no means an invariable rule. The inferior surface of the test is 
not unfrequently decorated with slightly raised costae, or with lines of exogenous granules 
radiating from the umbilicus ; and occasionally the sutures, superior and inferior, are 
limbate externally ; but with these exceptions the genus presents few examples of surface 
ornament. 
In no species of Discorbina has any evidence been found of the existence of true 
interseptal canals, but Carpenter has the following remark concerning Discorbina 
rimosa (and the observation holds equally true of Discorbina poly stomelloides) — “in this 
variety we find not only the Asterigerine flaps so much developed as to form a regular 
series of secondary chambers alternating with the primary, but we also find the 
exogenous deposit partially bridging over the superficial entrance to the interseptal 
fissures, dividing it into a row of little passages ; and thus is produced a sort of sketch of 
that intraseptal system of canals which we shall see to be more fully elaborated in Rotalia, 
and to attain its most complete development in Polystomella ” (Introd. Foram., p. 205). 
Parker and Jones have proposed a classification of the genus, under which it is 
divided into three sections, embracing respectively, — (l) the conical, (2) the vesicular, and 
(3) the outspread, more or less complanate forms. But the illustrative scheme which they 
furnish only includes sixteen species, and any attempt to adapt the same arrangement to 
a much larger series is attended with unsatisfactory results; indeed the authors themselves 
are careful to explain that “ the varieties are so intimately connected one with the other 
that the classification is little more than suggestive and provisional.” 
In one or other of its numerous modifications the genus Discorbina is found in almost 
every sea. Its known area of distribution extends from Novaya Zemlya and Davis Strait 
to the equator, and from the equator to Magellans Strait and Kerguelen Island. It is 
most common in the shallower zones of the ocean, becoming comparatively scarce at 200 
or 300 fathoms, and gradually less frequent down to 1000 fathoms, which is its approximate 
bathymetrical limit. The earliest appearance of the genus, geologically speaking, is 
towards the end of the Cretaceous period, and it recurs in almost every marine shallow- 
water deposit of the Tertiary and Quaternary epochs. 
Discorbina turbo, d’Orbigny, sp. (PL LXXXVII. fig. 8, ci.b.c.). 
Rotalia ( Trochulina ) turbo, d’Orbigny, 1826, Ann. Sci. Nat., vol. vii. p. 274, No. 29 ; — Modele, 
No. 73. 
„ turbo, Jones and Parker, 1860, Quart. Journ. Geol. Soc., voL xvi. p. 306. 
Discorbina turbo, Carpenter, 1862, Introd. Foram., p. 204. 
„ ,, Parker, Jones, and Brady, 1865, Ann. and Mag. Nat. Hist., ser. 3, vol. xvi. 
p. 30, pi. ii. fig. 68. 
Discorbina turbo, the type of the genus, has a stoutly-built subconical shell, with nearly 
