﻿VALVULINA. 



83 



rather than to the " Perforata " with the explanation that " it would perhaps be more 

 correct to assign to it an independent position as the connecting link between the two." 

 I am quite prepared to endorse this view and, as has been already stated in the 

 Introduction, would further add to the same intermediate category the Carboniferous 

 genus Endothyra and the Liassic type Involutina, although the relative positions of the 

 three must be left an open question pending more accurate researches on the latter type. 

 It is a well-ascertained fact that the freshly formed chambers in Valvulina sometimes 

 appear hyaline and perforate, though subsequently they become thickened with arenaceous 

 incrustation. Transparent sections show that in its best developed condition, notably in 

 some of the large specimens of Tertiary age, the shell has throughout a perforate basis 

 which has only become impervious by incrustation with extraneous matter ; and the 

 Carboniferous beds furnish at least one species, V. bulloides, with a not unfrequently 

 porous test. I have myself observed indications of the same thing in Involutina derived 

 from Liassic clays ; and, though the existence of a similar condition has not been demon- 

 strated in Endothyra, it is possible that this may be owing to the obliterating effects of 

 infiltration ; at any rate it is premature to say that it does not exist in any of the various 

 modifications of that type. 



In its distribution Valvulina has hitherto been regarded chiefly in the light of a 

 Cretaceous and early Tertiary type, at least it has not been supposed to extend further 

 back in geological time than the Cretaceous epoch. It has still living representatives in 

 some of the trochoid and plano-convex varieties, but, if we except the fine adherent 

 examples of V. conica, recently obtained from deep Atlantic soundings, they for the most 

 part bear the impress of a degenerate race. 



So far, however, from being subject to such limitations in point of time, it will 

 be gathered from subsequent pages that the genus furnished some of the most abundant 

 and characteristic species that are found in the microzoic rocks of the Carboniferous 

 period. 



Valvulina pal/Eotrochus {Ehrenberg) PI. IV, figs. 1 — 4. 



Tetrataxis conica, Ehrenberg, 1843. Bericht. d. k. Preuss. Akad., Jahr. 1843, 



p. 106. 



Textilaria PALyEOTROCHUS, Ehrenberg, 1854. Mikrogeologie, pi. xxxvii, § xi, figs. A' 



1—4. 



Tetrataxis conica, Id. Ibid., pi. xxxvii, § xi, figs. 7, 8. 



Involutina — Brady, 1871 (in Young and Armstrong's Catalogue). Trans. 



Geol. Soc. Glasgow, vol. hi, Suppl., p. 14. 

 Valvulina pal/eotrochus, Id., 1873. Ibid., vol. iv, pt. iii, p. 273. 



— — Id., 1873. Mem. Geol. Survey Scotland; Expl. Sheet 23, 



pp. 61, 95, &c. 



