286 FAMILY NUMMULINIDA. 



of this exogenous deposit into solid prolongations radiating from the margin of the spire. 

 I quite agree with Prof. Williamson (ex) in regarding all these as varieties of P. crisjxi, since 

 the specimens collected on our own shores exhibit all the modifications by which they are 

 severally distinguished, though in a less obvious degree. Certain of these varieties have a 

 tendency to become partially unsymmetrical, one side being quite flattened and exhibiting the 

 whole of the spire, but the septal plane and aperture being but little affected. This is espe- 

 cially the case' with the P. macella {Nautilus maceUiis of Fichtel and Moll). It is one of these 

 unsymmetrical forms which has been described by D'Orbigny (lxxiii) from the Chalk of 

 Maestricht under the name of Faiijasina carinata. For additional remarks on the reputed 

 species of PoJystomdIa, I may refer to the judicious criticisms of Messrs. Parker and Rupert 

 Jones (lxxviii). 



483. There is a group of forms, however, in which we meet with a greater departure 

 from the ordinary type of the genus ; the crenulations of the shell and the retral processes 

 of the sarcode-body lodged in them being altogether wanting, whilst the perforations in the 

 septal plane, instead of forming a series of isolated pores, run together into a continuous slit 

 or fissure along the margin of the included whorl, like that of Ojieradina and XuimnnUna. 

 The transition to this sub-generic type, which has been distinguished by the generic designa- 

 tion NoNiONiNA, is effected by the interesting form which was originally described by Fichtel 

 and Moll (xlv) as Naidllus striato-jmndalus, and which has been described and figured by Prof. 

 Ehrenberg (xl) from living specimens as Gcoponus slelln-horealis. This must be regarded as 

 in all essential characters a true Poli/stomella ; for, notwithstanding its minuteness as compared 

 with the larger forms of that type, it presents in a most complete form the canal-system by 

 which they are specially distinguished, a row of orifices of the diverging branches being 

 conspicuous externally (in favorable specimens) on either side of each septal band ; it has, 

 moreover, a solid umbihcal deposit, perforated by canals from which pseudopodia issue in its 

 living state; its septal aperture, though sometimes a single fissure, is very commonly sub- 

 divided by transverse bars of shell into a row of isolated pores ; and the whole shell is 

 covered with an exogenous deposit in the form of tubercles or minute granules, a feature 

 which is especially apparent in the variety distinguished by Schultze (xcvii) as P. r/ibba. 

 Moreover we have frequently an indication in this form (which seems like a reduced copy of 

 P. craticulata) of crenulations along the posterior margin of each of the segments marked out 

 by the septal bands. The Nautilus faha of Fichtel and Moll is another transitional form 

 between the typical PotystomeJla and the Nonioninc group ; of which last the Nautitus asteri- 

 zans of Fichtel and Moll may be considered a typical example. This derives its name from 

 the radiation of the exogenous deposits from the umbilical region along the septal bands ; a 

 feature which is still more pronounced in the N. limba of D'Orbigny (Modcles, No. 11), and 

 which is curiously modified with flaps in his N. stcltifera (v, plate iii, figs. 1, 2). The reputed 

 species of Nonionina need a careful investigation to ascertain how far they are conformable to 

 what has here been given (in conformity with the views of Messrs. Parker and Rupert Jones, 

 Lxxvin) as the true " idea " of a form which has hitherto been so vaguely defined as to 

 lead some systematists to propose the entire suppression of it as a generic type, whilst others 

 (ex) have proposed to include in it OpcratVma and Assilina. 



