GENUS OPERCULINA. 247 



peculiarities, whilst they indicate the relationship of Amphistegina to the Rotalines, differentiate 

 it in the like degree from the Nummulines. To the latter group, however, it presents so close a 

 conformity in the texture of its shell, in the complete investment of each convolution by that 

 which succeeds it, in the presence of alar lobes upon its upper as well as on its under surface, 

 and in its general tendency to bi-lateral symmetry, that we cannot hesitate in considering 

 it as most nearly related to tlie Nummulines ; its special affinities being to Nummlina its 

 general plan, and to the simple "nonionine" Poli/stomella in the exogenous deposit whicli 

 forms its umbilical "bosses." It may thus be considered in the light of a Nummuline 

 degraded to the Rotaline level ; just as the true BotaJia is a Rotaline elevated to the Nummu- 

 line level, so as to stand, in grade of development, considerably higher than Amplmtegina. 



429. GeofjrapJiical and Geological Distribution. — The range of this type through tropical 

 and sub-tropical regions appears to be very general, as it has been found in various parts of 

 the Indian Ocean, the great Polynesian area, and the West Indian seas ; the furthest limits to 

 which it is known to extend northwards are the Red Sea and the neighbourhood of the Canary 

 Islands ; while southwards it has not been traced further than New Zealand. It occurs in 

 greatest size and abundance at depths of from 15 to 50 fathoms ; but small specimens have 

 been brought up from abyssal soundings in the Red Sea. No true Amphistegina has yet been 

 discovered earlier than the Tertiary epoch ; and this genus does not seem to have become 

 abundant until after the period when the proper Nummuline type attained such an extra- 

 ordinary development, presenting itself especially in the Miocene and Pliocene deposits of 

 Bordeaux, Vienna, San Domingo, the Sub-Apennine group, Virginia, South Carolina, Alabama, 

 and New Zealand. 



Genus II. — Operculina (Plate XVII). 



430. Historg.— The genus Operculina was first created by D'Orbigny (lxix) for the recep- 

 tion of a type of Foraminiferous structure allied to N'lmmulites, but definitely (as he considered) 

 differentiated from it ; examples of this type, however, had been previously described by 

 Gronovius (l) and Schroter (xcv) under the generic designation A^autilus, and by Basterot 

 and Defrance (xxix) under that of Zenticulites, a name which has been applied to such a great 

 variety of dissimilar organisms that it cannot with any propriety be retained. According to 

 D'Orbigny (lxxiii), Operculina is distinguished from Nummulina by the extreme compression 

 of its discoidal shell, by the smaller number and more rapid increase of its whorls, by tlie 

 non-investment of the earlier whorls by the later, so that the former remain apparent on 

 both sides through the whole of life, and by the form of the aperture, which is a transverse slit 

 in Nummulina but triangular in Operculina. The genus, as characterised by D'Orbigny, has 

 been adopted by -many subsequent systematists, as Bronn and Pictet ; and it is also admitted 

 by D'Archiac and Haime (i), who, however, seem to have clearly perceived the fallacy of 

 D'Orbigny's description of the convoluiions as non-embracing, and of the aperture as 

 triangular, since they consider it to be distinguished from Nummulina only by the depression ot 

 its form, the small num.ljer of its convolutions, and the rapid increase in breadth of the last 

 whorl, which (according to them) constantly remains open, whilst that of Nummulina always 



