212 FAMILY GLOBIGERINIDA. 



Gejius XVI.— RoTALiA (Plate XIII, figs. 7—9; Williamson, Figs. 90—92, 106—108). 



369. External Characters. — It is in the true RofalicB, of which R. Beccarii is the typical 

 example, that we meet with the highest development of the Rotaline type, and the nearest 

 approximation to the Nummuline. This approximation is shown in its shell-substance, which 

 is of unusually fine texture, its pores being as minute and as closely set as they are in Opercu- 

 lina or Nummdina (Plate XIII, fig. 8). It is further marked in the general compactness of 

 the shell, and in the mode in which it is strengthened and completed by an " intermediate 

 skeleton." But it is especially manifested in this important feature, — that every ciiamber is 

 completely surrounded by its own proper wall, no part of its boundary being borrowed from 

 that which belongs to the penultimate segment. Hence every septum dividing adjacent 

 segments is double instead of single ; and its substance rather resembles that of the interme- 

 diate skeleton than that of the ordinary chamber-wall, being entirely destitute of the minute 

 porosity which characterises the latter. In this respect, therefore, there is an important 

 advance in that development which consists in progressive differentiation. In the ordinary 

 British form of P. Beccarii, which is accurately described and figured by Prof Williamson 

 (ex, p. 49, Figs. 90 — 92), the shell has a rather depressed turbinoid form, with a 

 rounded margin ; and is composed of a considerable number of segments progressively 

 increasing in size, disposed with great regularity, and having their apposed surfaces 

 closely fitted to each other. The whole of the spire from its commencement is visible on the 

 upper surface ; whilst on the lower only the last convolution shows itself, the segments being 

 arranged around a deep umbilical vestibule, which is filled up with a column of homogeneous 

 semi-crystalline shell-substance. This column sometimes projects as an umbo from the 

 surface, and is clustered over with granulations and incrustations of the like material; similar 

 granulations extend along the umbilical margins of the segments, which are frequently a good 

 deal separated from each other, the intermediate spaces being sometimes filled up with tiic 

 semi-crystalline substance resembling that of the umbilical column. Owing to the somewhat 

 ventricose form of the segments, their lines of junction along the whole spire would be deeply 

 furrowed, were it not that the depressions are partly filled up by deposits of the like substance, 

 which strongly mark the septal lines by the difference of their aspect from that of the inter- 

 vening porous chamber-walls. The septal plane is distinguished by its translucency ; and the 

 aperture is usually a neat arched slit, which is much more nearly medial than in the Rotalincs 

 generally, being very little beneath the contiguous peripheral margin of the penultimate 

 convolution'. In some varieties, however, the aperture has a notch in the umbilical margin of 

 the septal plane. The diameter of this shell, which is an inhabitant of moderately deep 

 water, is usually from l-2.5th to l-30tli of an inch. 



370. A feeble shallow-water or littoral variety of this type has been described and figured 

 by Prof. Williamson (ex, p. 54, Figs. 106 — 108) under the designation R. uifida ; here the 

 segments become quite " flush," so that the surface of the cone is smooth ; the walls of the 

 chambers are very thin and translucent, and there is but very little exogenous deposit either 

 along the septal bands or in the umbilical vestibule. A like deficiency of exogenous deposit 

 is observable in the large smooth R. ammonifornm (D'Orb.) of the Rimini shore, which lias 



