﻿TROCHAMMINA. 



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In the recent condition Troclmmmina incerta is very uniform in its general character 

 and appearance ; and, as stated by Prof. Williamson (' Monogr./ p. 93), it is widely 

 distributed but nowhere abundant. This however cannot be said of its occurrence in 

 the Carboniferous rocks. Wherever it exists as a Palaeozoic fossil, it appears in large 

 numbers, and the specimens present a correspondingly wide range of variation in minor 

 characters. Many specimens are just such as might be dredged at the present day on 

 our own shores, consisting of five or six convolutions in one plane of a non -septate 

 tube, the convolutions nearly uniform in breadth, and the tube having an approximately 

 circular transverse section. The examination of a large number of individuals reveals 

 many little modifications of these simple typical characters. Sometimes the number of 

 convolutions is smaller and their width greater, forming a test of similar diameter and 

 without increase of thickness (PI. II, fig. 12), and in such the tube presents a long oval, 

 instead of a circular transverse section. Other examples show a tendency in the 

 successive convolutions each to embrace, to a limited degree, that immediately within it, 

 and the section of the tube is then more or less crescentiform. In some of the larger 

 complanate shells (fig. 11) the spiral tube increases in width with each succeeding 

 circlet. Lastly, it is not at all uncommon to find the shell-wall thickened, especially 

 near the centre of the disc, the excavated sutural line filled up, and the test assuming 

 thereby a more or less lenticular or bi-convex figure. In these instances the calcareous 

 cement is largely in excess of arenaceous material, the surface of the test is nearly 

 smooth, and even permits, by a sort of transparency, the course of the spiral cavity in the 

 interior to be traced (PI. I, fig. 13). These modifications, in addition to many 

 irregularities in external contour, arise from what may be regarded as accidental circum- 

 stances, and present no ground for specific or varietal subdivision. 



It seems necessary to make passing allusion to some of the Mesozoic Trochammina 

 figured by M. Terquem under the generic terms Lwohdina and Cornuspira. M. Terquem 

 appears to stand alone amongst students of Poraminifera in his non-acceptance of shell- 

 texture as the basis of the primary division of the order. Thus, Cornuspira with its 

 imperforate porcellanous shell, Trochammina with its imperforate arenaceous test, and 

 Spirillina with its brilliantly hyaline, porous walls, the isomorphic genera of the three 

 primary groups of Reticularian Rhizopods, are regarded by him as one genus. M. Terquem 

 has been good enough to send me, for purposes of comparison, type specimens of several 

 of- the species of Involutina described in his memoirs on Liassic and Oolitic Poraminifera, 

 together with notes upon them indicating some change in his views concerning that 

 genus ; also specimens of the Cornuspira represented in pi. xxv of his third ' Memoir on 

 the Poraminifera of the Oolite,' viz. C. granulosa, C. infraoblithica, C. punciulata, 

 C. concava, C. aspera, and C. occlusa, figures 12 to 20 respectively. After a very 

 careful examination of this series I may confess that I find nothing in their characters 

 that seems to me to justify varietal, much less specific separation one from another. In 

 his text, p. 242, the author objects to the generally received definition of the genus 



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