330 
THE VOYAGE OF H.M.S. CHALLENGER. 
This generic or sub-generic group includes all the non-septate tubular Trochamminmce. 
In its typical condition the test is spiral ; and the various subordinate forms differ from 
each other chiefly in the manner in which the convolutions are arranged. The tube itself 
is sometimes of nearly uniform diameter from end to end, but more frequently increases 
in width more or less with each convolution. 
The genus Ammodiscus is represented in one form or other in almost every part of 
the world, and at every depth at which the sea- bottom has been explored, from the 
estuarine shallows of our own coast down to 3000 fathoms or more ; and its geological 
history extends as far back as the earliest part of the Carboniferous epoch. 
Ammodiscus incertus, d’Orbigny, sp. (PI. XXXVIII. figs. 1-3). 
Qperculina incerta, d’Orbigny, 1839, Forain. Cuba, p. 71, pi. vi. figs. 16, 17. 
Orbis infimus, Strickland, 1848, Quart. Journ. Geol. Soc., vol. ii. pi. xxx. fig. a. 
Spirillina arenacea, Williamson, 1858, Rec. For. Gt. Br., p. 93, pi. vii. fig. 203. 
Trochammina squamata incerta, Jones and Parker, 1860, Quart. Journ. Geol. Soc., vol. xvi. p. 304. 
„ incerta, Carpenter, 1862, Introd. Foram., p. 141, pL xi. fig. 2. 
,, squamata, var. incerta, Parker and Jones, 1862, Ibid. Appendix, p. 312. 
Involutina silicea, Terquem, 1862, Mem. Acad. imp. Metz, 1860-1, p. 450, pi. vi. fig. 11. 
„ aspera, Id. 1863, Ibid. 1862-3, p. 221, pi. x. fig. 21. 
Cornuspira hoernesi, Karrer, 1866, Sitzungsb. d. k. Ak. Wiss. Wien, vol. lii. p. 495, pi. i. fig. 10. 
Cornuspira oolithica, Sell wager, 1867, In Waagen’s Ueber die Zone des Amm. sowerbyi, 
vol. i. pt. iii. p. 655, pi. xxxiv. fig. 4. 
Trochammina incerta, Brady, 1876, Monogr. Carb. and Perm. Foram., p. 71, pi. ii. figs. 10-14. 
Ammodiscus miocenicus, Karrer, 1877, Geol. K. F.-J. Wasserleitung, p. 372, pi. xvi. a., fig. 2. 
Ammodiscus incertus , Berthelin, 1878, Foram. de Bourgneuf et Pornicbet, p. 23, No. 17. 
Trochammina incerta, Haeusler, 1882, Ann. and Mag. Nat. Hist., ser. 5, vol. x. p. 52, pi. iii. 
figs. 1-4. 
Test spiral, discoidal, thin ; peripheral edge rounded ; consisting of numerous narrow 
convolutions on one plane of a non-septate tube of slightly increasing diameter. Aper- 
ture usually formed of the open unconstricted end of the tube. Texture very finely 
arenaceous, exterior smooth ; colour (of recent specimens) ruddy brown. Diameter, y^th 
to -Jth inch (0'25 to 2'8 mm.). 
Owing to the very frequent occurrence of Ammodiscus incertus in the fossil condition, 
and to the extent of its geological range, few species of Foraminifera have been described 
under so many different names. Of these only the more important appear in the above 
synonymy, and a much longer list might easily be compiled. 
In an organism so simple that the essential characters of the test may be summed up 
in half a dozen words — a smooth arenaceous planospiral tube — the points open to varia- 
tion are necessarily few ; in fact they are almost limited to the size of the disk, the 
diameter of the tube, the shape of its transverse section, and the number of convolutions ; 
and all these are more or less dependent on one another. 
