POLYttTOMELLA 827 



with a * canal system ' for its nutrition. It is true that these cha- 

 racters are also exhibited in the highest of the Rotaline series, whilst 

 they are deficient in the genus Amphistegina, which connects the 

 Nummuliiie 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 AmphiAtgina there is a reversion to the 

 Rotalian type in the turbinoid form of its spire, as in the characters 

 already specified, 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 family. Notwithstanding the want of symmetry of 

 its spire, it accords with Operculina and Nummulites 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 dis- 

 placed 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 spire, and made this the 

 character of a separate order. 1 



The existing Nummulinidce are almost entirely restricted to 

 tropical climates ; but a beautiful little form, Polystomella crispa, 

 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. The peculiar surface- 

 marking shown in the figure consists in a strongly marked 

 ridge-aiid-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 communication which 

 the sarcode-body of any segment has with the exterior being 

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

 row 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 of the plication of 

 the shelly wall comes to be understood when we examine the con- 

 formation of the segments of the sarcode-body, which may be seen 

 in the common Polystomella 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. 620) 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 dead shells 

 which have undergone infiltration with ferruginous silicates. 2 Here 



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). 



2 It was by Professor Ehrenberg that the existence of such ' casts ' in the Green- 

 sands of various geological periods (from the Silurian to the Tertiary) was first 

 pointed out, in his memoir ' Ueber den Griinsand und seine Erlauterung des 



