746 
THE VOYAGE OF H.M.S. CH ALLEN GEE. 
nature of the aperture, which in Operculina consists of a transverse arched fissure at the 
inner margin of the terminal segment. In Heterostegina, on the other hand, the 
chamberlets of which each chamber is composed have no direct communication with each 
other, but each chamberlet has an orifice on the outer septal plane ; and the row of pores 
so formed on the face of the final chamber constitutes the general aperture. The canal 
system resembles that of Operculina. 
The genus Heterostegina inhabits the tropical portions of the Atlantic, Pacific, and 
Indian Oceans, whilst in the Red Sea it is found as far north as the Gulf of Suez. Its 
bathymetrical range extends from the littoral zone to a depth of about 600 fathoms. 
Fossil representatives of the genus occur in Tertiary deposits of almost every age from 
the Eocene forwards. 
Heterostegina depressa, d’Orbigny (PI. CXII. figs. 14-20). 
Heterostegina depressa, d’Orbigny, 1826, Ann. Sci Nat., vol. vii. p. 305, pi. xvii. figs. 5-7; — 
Module, No. 99. 
„ antillarum, Id. 1839, Foram. Cuba, p. 121, pi. vii. figs. 24, 25. 
„ helvetica, Kaufmann, 1867, Geol. Beschreib. des Pilatus, p. 153, pi. ix. 
figs. 6-10. 
The Challenger collections have furnished a smaller number of specimens of 
Heterostegina than might have been expected, and their study has added but little to 
our knowledge of the type. Whether the living representatives of the genus are all 
referrible to a single species is a question which still remains undetermined. The 
immature shells present themselves under two apparently very distinct forms, in 
one of which (figs. 17, 18) the test is compressed and explanate, and the chambers 
subdivided from the earliest stages; whilst the contour of the other (figs. 19, 20) 
is biconvex, the convolutions are embracing, and the subdivision of the chambers only 
commences after one or two convolutions have been formed. 
The points in which the adult shells exhibit the greatest amount of variability are — 
the degree of convexity of the two faces of the test ; the greater or less rapidity of the 
widening of the spire ; the development and thickening of the marginal cord ; and the 
greater or less regularity, as well as the amount of external limbation, of the primary and 
secondary septal lines. Specimens, to all appearance fully grown, vary considerably also 
in dimensions, and occasionally attain a diameter of half an inch (12 '6 mm.) or even more. 
Heterostegina depressa has been taken amongst the Cape de Verde Islands, in shore- 
pools, and at a depth of 11 fathoms ; amongst the West Indies ; in the South Atlantic, 
off Pernambuco, 350 fathoms; in the Red Sea, 30 fathoms; on the shores of the 
Seychelle Islands, Madagascar, and Ceylon ; at eleven Stations amongst the islands of 
the South Pacific, 6 fathoms to 620 fathoms ; in the Chinese Sea ; and on the coral-reefs 
of the Sandwich Islands, 40 fathoms. 
