70 CARBONIFEROUS AND PERMIAN FORAMINIFERA. 



General characters. Test free or attached ; very variable in form, consisting of 

 one or many chambers : thin in substance, texture arenaceous, the sandy constituents 

 embedded in a calcareous, often more or less ferruginous cement, and not projecting 

 above the surface of the shell, which is nearly smooth. Polythalamous varieties are 

 without proper septa, the division into chambers being effected by the constriction or 

 of the primary shell-wall. 



As already stated the genus Trochammina, in its structural relations, stands between 

 Lituola on the one hand, and Involutina, Valmdina, and Endothyra on the other. The 

 division of the series into these four quasi-generic groups is to be accepted as a convenient 

 means of arranging, in something like intelligible sequence, a large number of varietal 

 and subvarietal forms which could not be dealt with otherwise not as the expression of 

 a classification founded on morphological characters capable of definition in accurately 

 distinctive terms. 



Comparing Trochammina with Lituola the thick test and labyrinthic interior of the 

 latter type are sufficiently distinctive, but the minute structure of the shelly investment is 

 also different in the two genera. The true Lituolcs are ordinarily more or less rough 

 externally from the excess of relatively large sand-grains employed in the building-up of 

 the test, whilst the Trockammince with smaller and less angular constituent grains (PL II, 

 fig. 14), and a much larger proportion of calcareous cement, have a comparatively smooth 

 exterior surface. Whilst, therefore, both have composite tests, Lituola may be spoken of 

 as " sandy," and Trochammina, in distinction, as " cemented," in texture. Trochammina 

 inhabits shallower water than Lituola, and in estuaries and brackish pools its test 

 becomes thinner and less calcareous, the mineral constituents being replaced by a sort of 

 chitinous membrane. 1 The distinction between Trochammina and the Liassic genus 

 Involutina of Terquem is still less easy to reduce to words. In general terms Involutina 

 (accepting I. liassica as its representative) approaches much more nearly in structural 

 features to the lower Rotalians. Its test is often a good deal thickened by the deposit of 

 nearly homogeneous shell-substance and, occasionally at least, it shows, like Valvulina, a 

 perforate primary shell-wall. 



It is amongst the small recent specimens living in comparatively shallow water that 

 the chief difficulties in separating the genera Lituola, Trochammina, and Valvulina are 

 experienced ; indeed, as has been stated both by Messrs. Parker and Jones 2 and myself, 8 

 a series of individuals referable to these three types may be readily got together forming 

 a complete chain, showing no break or missing link to warrant specific, still less generic 

 separation. It is true that in a chain so arranged many of the links might be supplied 



1 Vide ' Ann. and Mag. Nat. Hist.,' ser. 4, vol. vi, p. 290, Trochammina iiiflata, var. macrescens, pi. xi, 

 fig. 5. 



2 ' Ann. and Mag. Nat. Hist.,' ser. 4, vol. iv, p. 391. 



3 Ibid., ser. 4, vol. vi, p. 290. 



