yiJ THE MICROSCOPE AND ITS REVELATIONS. 



type in tlie turbinoid form of its spire, as in the characters already speci- 

 fied, although, its general conformity to the Nummuline type is such as 

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

 Notwithstanding the want of symmetry of its spire, its 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'Orbignv, supposing 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. 1 



487. The existing Nummulinida are almost entirely restricted to tropi- 

 cal 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 



Section of Fusulina-IAinesione. 



common on our own coasts. The peculiar surface-marking shown in the 

 figure consists in a strongly marked ridge and furrow plication of the 

 shelly wall of each segment along its posterior margin; the furrows being 

 sometimes so deep as to resemble fissures opening into the cavity of the 

 chamber beneath. No such openings, however, exist; the only com- 

 munication which the sarcode-body of any segment has with the exterior, 

 being either through the fine tubuli of its shelly walls, or through the 

 TOW of pores that are seen in front view along the inner margin of the 

 septal plane, collectively representing a fissured aperture divided by 

 minute bridges of shell. The meaning the plication of the shelly wall 

 comes to be understood, when we examine the conformation of the seg- 

 ments of the sarcode-body, which may be seen in the common Polysto- 

 mella crispa by dissolving away the shell of fresh specimens by the action 

 of dilute acid, but which may be better studied in such internal casts (Fig. 

 332) of the sarcode-body and canal-system of the large P. craticulata of the 

 Australian coast, as may sometimes be obtained by the same means from 



1 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 

 (published by the Ray Society). 



