190 FAMILY GLOBIGERINIDA. 



were not differentiated by him from other types associated under the same designation. This 

 differentiation was first made by Defrance (xxix), to whom we owe the estabhshment of the 

 genus Textularia, which has been adopted by all subsequent systematists. "With this genus 

 we find the Bigenerina, GemmuUtia, Gaudry'uta, Vcrneuiliiia, and Vithmliiia of D'Orbigny (the 

 last being the Grammostomum of Ehrenberg) to be not less intimately connected by gradational 

 links than are the thirteen reputed genera which we have reunited under the genus Noclosarina ; 

 and we therefore regard them as merely varietal forms of Texhihria. The Candeina of 

 DOrbigny appears to have more claim to rank as a sub-genus. 



317. External Characters The shell of Textularia essentially consists of a binary series 



of segments arranged symmetrically on the two sides of a longitudinal axis ; the segments of 

 one side alternating with those of the other, and each segment communicating with the 

 segments anterior and posterior to it on the opposite side (Plate XII, fig. 14). As the size 

 of the segments usually increases progressively, the outline of the shell is generally more or 

 less triangular ; the apex of the triangle being formed by the first segment, and its base by the 

 last two. Sometimes, however, the later chambers do not exhibit any progressive increase, so 

 that the previously diverging sides of the triangle become parallel ; and sometimes they even 

 undergo a progressive decrease, so that the sides converge again. The two lateral faces of the 

 shell usually show more or less of compression ; being sometimes nearly flat, with rounded mar- 

 gins, so that its transverse section is more or less oblong (ex. Figs. 162, 163, 168) ; whilst in 

 other instances the shell is more compressed, but is most prominent along the axial line, thinning 

 away at the margins (ex. Figs. 158, 159, 164, 165), which may even be sharply carinated, as in 

 the T. carinata of D'Orbigny. Sometimes, on the other hand, the two lateral faces, instead of 

 being compressed, are so rotund that the transverse section becomes nearly circular, so that 

 the form of the shell is almost conical (ex. Figs. 160, 161). The mode in which the segments 

 are applied one to the other, also, is subject to considerable variation. In some instances they 

 are almost spheroidal in shape, and are merely flattened against each other where they come 

 into mutual contact (ex, Figs. 162, 168) ; but in general they are more or less depressed and 

 elongated ; and whilst the direction of their longer axes seems to be typically at right angles, 

 or nearly so, to the axis of growth (ex. Fig. 158), it is sometimes considerably inclined to this 

 (ex, Figs. 164, 166). It is usually in those very compressed forms which are thinnest at the 

 margins, that the obliquity of the long axes of the segments is the greatest ; and sometimes 

 the outer margin of each chamber is prolonged into a sort of spine (ex, Figs. 166, 167). 

 In these also we often find the inner margin of each segment prolonged across the 

 longitudinal axis, so as to interdigitate with the alternate segments on the other side. 

 Between all these forms there is a completely gradational series, as Professor Williamson has 

 already pointed out (ex, p. 75). Again, some Textularia commence with a flat Operculine 

 spire, and subsequently change to a liitjii one ; just as certain small long-spired Gasteropods 

 begin like a Planorbis. Furtlier, in certain attenuated Textularia; from the Red Sea, the axis 

 of growth slowly twists upon itself, so that the later pairs of alternating segments are at right 

 angles with the earlier. Thus it is clear that in this type there is no less variety in regard to 

 external configuration, than we have seen to exist elsewhere. — The aperture is very constant 

 as to form and position, being a crescentic slit in the inner wall of each chamber close to its 

 junction with the antecedent segment of the opposite side (ex, Figs. 159, 161, 163, 165, 167). 



