GENUS AMPIIISTEGINA. 241 



419. It is a point of no slight interest in regard to the Geographical Distribution of this 

 family, that it is scarcely at all represented at the present time in the seas of temperate 

 or arctic zones, being almost exclusively confined to tropical or sub-tropical regions. The 

 few examples that occur nearer to the poles than to the equator are the least developed of 

 their respective genera, in regard both to size and to complexity of structure ; whilst the 

 larger and more developed forms of those genera, like the great J/eferosfr</ina and the 

 gigantic C//duch/pe/fs, are only to be met with between or near the tropics. This fact is one 

 of considerable interest and importance in relation to the question of the climate tliat 

 prevailed in Europe during the earlier portion of the Tertiary Epoch. 



Genus I. — Amphistegina, (Plate XIII, figs. 22 — 29.) 



420. History. — This genus was first constituted by M. D'Orbigny in 1825 (lxix) for the 

 reception of a type of Foraminiferous shells whicli does not seem to have been previously 

 distinguished by systematists who have given their attention to this group, in consequence, 

 it may be, of its limitation to the seas of warm latitudes. Notwithstanding the very close 

 relationship which it will be presently shown to bear to JVummulinida in general plan of struc- 

 ture, it was considered by M. D'Orbigny to depart from them in a character which he 

 regarded as of such fundamental importance as to serve for the basis of a distinct order, that 

 oi Enfomosteffues, of which the following is his most recent definition (lxxiv) : " Animal com- 

 pose de segmens alternes, formant une spirale. Coquille composee de loges empilees ou 

 superposees sur deux axes alternant entre elles, et s'enroulant en spirale." The mode of 

 increase in these shells, he elsewhere says (lxxiii, p. 199), presents a singular mixture of 

 that of the Eiiallostegiies with alternating chambers, and of the spiral involution of the Heli- 

 costefftces. I have already pointed out that such a combination does actually exist in Cassi- 

 diclina (^ 3.38), which, so far as I know, is the only type that exhibits it ; but the alternation 

 of chambers supposed by M. D'Orbigny to characterise Amphistpgiiia will be found not to have 

 a real existence. His account of it, however, has been taken upon trust by all those Natu- 

 ralists who have adopted his system ; the only investigator, so far as I am aware, who has 

 questioned his views being Prof. W. C. Williamson, who, in his extremely valuable memoir 

 " On the Minute Structure of the Calcareous Shells of some Recent Species of Foraminifera" 

 (cviii), gave an account of the Amphisieyhta fj'ibbosa of the West Indian seas, which, as far as 

 it goes, is extremely accurate. He rightly apprehended the fact of its close conformity to 

 the Nummuline type, save in the fact of the want of symmetry which is occasioned by the 

 turbinoid declination of its spire ; but he failed to detect that peculiar " asterigerine" arrange- 

 ment of the alar prolongations on the under side, the misinterpretation of which seems 

 to have been the source of M. D'Orbigny's erroneous conception of the essential cha- 

 racters of this type. Tlie elucidation of the real nature of these is due to Messrs. Parker and 

 Rupert Jones, who have kindly furnished me with the materials on which the following 

 account of it is based.* 



* In the tliird series of my " Researches on the Foraminifera" ('Phil. Trans./ 1859), I described 

 under the name of Amphistegina Cumbigii what I am now convinced to be a Nummuliiia, since it is not 

 only perfectly symmetrical, but is destitute of the " astral lobes" which characterise all true Amphistegina. 



31 



