REPORT ON THE FORAMINIFERA. 
741 
Amphistegina gibbosa, Williamson, 1851, Trans. Micr. Soc. Fond., ser. 1, vol. iii. p. 110, pi. xvii. 
figs. 1, 2. 
„ lessoni, Parker, Jones, and Brady, 1865, Ann. and Mag. Nat. Hist., ser. 3, 
vol. xvi. p. 34, pi. iii. fig. 92. 
„ semicostata, Kaufmann, 1867, Geol. Beschreib. desPilatus, p. 149, pi. viii. fig. 18. 
„ lessonii, Moebius, 1880, Foram. von Mauritius, p. 99, pi. x. figs. 10-14 ; pi. xi. 
figs. 1-3. 
„ parisiensis, Terquem, 1882, Mem. Soc. g<iol. France, ser. 3, vol. ii. Mem. III. 
p. 124, pi. xiii. fig. 3, a.b. 
Thick forms, still more inequilateral, sometimes dome-shaped, fig. 7. 
Amphistegina mamillata, d’Orbigny, 1846, For. Foss. Yien., p. 208, pi. xii. figs. 6-8. 
„ rugosa, Id. Ibid. p. 209, pi. xii. figs. 9-11. 
Hemistegina rotnla, Kaufmann, 1867, Geol. Beschreib. des Pilatus, p. 150, pi. viii. fig. 19. 
It is impossible to separate, even by varietal characters, the various forms of A mphis- 
tegina represented by the drawings PI. CXI. figs. 1-7. 
The typical aspect of Amphistegina lessonii is that represented by d’Orbigny in the 
plates accompanying the “ Tableau Methodicjue,” and well rendered in fig. 3 of our 
illustrations. D’Orbigny ’s Model of the same species (No. 98) portrays a much thicker 
shell with fewer segments, resembling our fig. 5. The Model, No. 40, named Amphis- 
tegina vulgaris, is founded upon a more outspread and less regularly constructed 
specimen, something like our fig. 2, but thicker just at the centre. 
Owing to the peculiar form and disposition of the segments, the test of Amphistegina 
is almost necessarily more or less inequilateral. The superior face is nearly always more 
convex than the inferior, but the degree of asymmetry differs in different individuals, 
and specimens like fig. 7, in which the superior side is highly convex and the inferior 
nearly flat, are by no means unfrequent. Such forms constitute the Amphistegina 
mamillata of d’Orbigny, and the Hemistegina rotula of Kaufmann. 
Wherever Amphistegince are abundant, and at the shallow- water margins of warm 
seas they sometimes form the principal constituent of the bottom-sand, all these 
variations of the typical structure are met with, together with every intermediate 
condition ; and in such profusion as to make it impossible to conceive that the differences 
between the extremes of the series are anything more than individual peculiarities. 
Within certain limits as to depth, Amphistegina lessonii is generally distributed over 
the tropical portions of the Atlantic, Indian, and Pacific Oceans. In the Atlantic it 
reaches northward as far as Bermuda and Teneriffe, and* in the Red Sea to the Gulf of 
Suez ; but, with these exceptions, there is no record of its occurrence outside the tropical 
zone. It is commonest on bottoms of less than 30 fathoms depth, but is found with some 
frequency down to 300 or 400 fathoms, below which it is rare. The occurrence of 
specimens in deeper water has been noted at two Stations in the North Atlantic, 1070 
and 1750 fathoms respectively ; at one in the South Atlantic, south-east of Pernambuco, 
G75 fathoms ; and at one in the South Pacific, off Tahiti, 620 fathoms. 
(ZOOL. CHALL. EXP. PART XXII. — 1884.) 
Y 94 
