﻿TROCHAMMINA. 



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. It may be doubted whether Trochammina centrifuga is more than a varietal modi- 

 fication of Tr. incerta, but it appears to be a well-marked form, and there is something 

 distinctive in its mode of occurrence. Wherever it is found Tr. incerta is also present ; 

 but, on the other hand, out of fifty localities for the latter species, Tr. centrifuga has only 

 been noticed in ten or twelve. In the localities where it does occur it generally exists 

 in large numbers, not as a rarity. In applying to the form a distinguishing name 

 I have, as in many other cases, been guided by convenience rather than by those strict 

 zoological rules which seem inapplicable to the lowest types of animal life. 



The test of Trochammina centrifuga begins its growth on the same plan as Tr. 

 incerta, that is as a flat spire ; but after a number of convolutions are formed, the course 

 of the tube changes abruptly from spiral to rectilinear, and a straight arm of greater or 

 less length is added to the previously discoidal test. The number of convolutions is very 

 variable ; — sometimes the primary portion consists of one or two turns of a wide, flat, 

 irregular, spiral tube, at others (as in the section PI. II, fig. 20) four or five revolutions 

 may be observed, comparatively regular in outline and gradually increasing in width. 

 In general contour the discoidal portion differs much in different specimens ; sometimes 

 it is nearly circular, often angular or altogether irregular ; and the lateral surfaces are 

 either bi- convex, depressed, or slightly concave with excavated umbilicus. 



The rectilinear arm varies much in length. In what may be assumed to be adult 

 specimens it projects to an extent about equal to the diameter of the disc. The end of 

 the tube, which is usually open and unconstricted except by occasional slight thickening 

 at the rim, forms the aperture. 



The external inequalities of the tube led at first to the supposition that there existed 

 some sort of spurious or imperfect septation, and with this idea Mr. Charles Moore's 

 specimens were provisionally placed with the Involuting, 1 but horizontal sections show 

 that the superficial irregularities are not connected with modifications of the shell-wall on 

 its interior, and that the test is really, as shown in PI. II, fig. 20, a non-septate tube. In 

 transferring the species to the genus Trochammina it became necessary to alter its name, 

 the term " incerta' being already occupied in that group, hence the adoption of the 

 present name in the Catalogue of the Foraminifera of the Carboniferous beds of Lanark- 

 shire, compiled for the " Memoirs of the Geological Survey of Scotland." 



Distribution. — In the Lower Carboniferous Limestone beds (Scar) of England and of 

 Scotland, Trochammina centrifuga is of very rare occurrence, having been noticed in only 

 one locality in each country. In the Upper portion of the series, both in England 

 (Yoredales) and in Scotland, it is much more frequently found, ten out of the twelve 

 recorded localities pertaining to these higher groups of limestones. 



1 ' Report of the British Association,' loc. cit. 



