506 foraminifera: — nummulinida. 



certainty, not only that Fusulina is tubular, but that its tabu- 

 lation is of the large coarse nature that marks its affinity rather 

 to the Rotaline than to the Nummuline series. — This type is of 

 peculiar interest as having long been regarded as the oldest form 

 of Foraminifera, which was known to have occurred in sufficient 

 abundance to form Rocks by the aggregation of its individuals. 

 It will be presently shown, however, that in point both of anti- 

 quity and of importance, it is far surpassed by another (§ 397). 



389. Nummulinida. — All the most elaborately constructed, and 

 the greater part of the largest, of the Vitreous Foraminifera belong 

 to the group of which the well-known Nummidite may be taken 

 as the representative. Various plans of growth prevail in this 

 Family ; but its distinguishing characters consist in the complete- 

 ness of the wall that surrounds each segment of the body (the 

 septa being double instead of single as elsewhere), the density and 

 fine porosity of the Shell-substance, and the presence of an Inter- 

 mediate Skeleton, with a Canal-system for its nutrition. It is 

 true that these characters are also exhibited in the highest of the 

 Rotaline series (§ 387), whilst they are deficient in the genus Amphi- 

 stegina, which connects the Nummuline series with the Rotaline ; 

 but the occurrence of such modifications in their border-forms is 

 common to other truly Natural groups. With the exception of 

 Amphistegina, all the Genera of this family are symmetrical in 

 form ; the spire being Nautiloid in such as follow that plan of 

 growth, whilst in those which follow the Cyclical plan there is a 

 constant equality on the two sides of the median plane : but in 

 Amphistegina there is a reversion to the Rotalian type in the 

 Turbinoid form of its spire, as in the characters already specified, 

 whilst its general conformity to the Nummuline type is such as to 

 leave no reasonable doubt as to its title to be placed in this family. 

 Not-withstanding the want of symmetry of its spire, it accords 

 with Operculina and Nummulina in having its chambers extended 

 by ' alar prolongations ' over each surface of the previous whorl ; 

 but on the under side these prolongations are almost entirely cut 

 off from the principal chambers, and are so displaced as apparently 

 to alternate with them in position ; so that M. D'Orbigny, sup- 

 posing them to constitute a distinct series of chambers, described 

 its plan of growth as a biserial spiral, and made this the character 

 of a separate Order.* 



390. The existing Nummulinida are almost entirely restricted 

 to tropical climates ; but a beautiful little form, the Polystomella 

 crispa (Plate xv., fig. 16), the representative of a Genus that 

 presents the most regular and complete development of the Canal 

 system anywhere to be met with, is common on our own coasts. 



* For an account of this curious modification of the Nummuline plan 

 of growth, the real nature of which was first elucidated by Messrs. 

 Parker and Rupert Jones, see the Author's " Introduction to the Study 

 of the Foraminifera " (Ray Society). 



