164 FAMILY LA.GENIDA. 



the perfectly straight Noclosaria, and the most arcuate Dentalinn. On the other hand, an 

 increased obhquity of the segmental divisions, accompanied with a lateral compression of the 

 shell, and with a greater or less degree of curvature of its axis, constitutes the varietal 

 modification to which the generic name Var/uudina has been given (ex, figs. 45 — 48). When 

 this form becomes extremely flat and broad, it receives the name Planularia. 



251. The form of the aperture, again, is often modified in accordance with that of the 

 septal plane ; and on this modification (of which we have seen marked examples in Miliola and 

 in the " dendritine" variety of Peneroplis) other genera have been founded. Thus Lingulina 

 (Plate XII, fig. 1 ) is nothing but a compressed Noclosaria, whose transverse section is oval, and 

 whose aperture (fig. 1, a) has undergone a corresponding elongation, sometimes becoming 

 crescentic or reniform. The low value of this modification as a differential cliaracter is 

 curiously evinced by the fact, that in the L. mutabilis of D'Orbigny, the last chamber, instead 

 of being compressed and bicarinated like those which preceded it, is triangular and tri- 

 carinated, and is furnished with a triangular aperture. In the various forms of Li/ii/ulina, 

 again, it is curious to see how precisely the superficial inequalities of Nodosaria and Cnstellaria 

 repeat themselves ; the costate varieties usually having the ribs of the two margins raised into 

 carinas. — The varietal modification on which the generic term Bimulina has been conferred by 

 D'Orbigny, is merely a compressed or vaginuline form of Dentalina, in which- the aperture, 

 instead of being circular and nearly central, is an elongated fissure extending through a large 

 part of the convex side of the anterior wall of the chamber (Plate XII, fig. 4). — And the varieties 

 of the Cristellarian type in which the aperture, instead of being round, is triangular (Plate XII, 

 fig. 3, «), have been distinguished by the generic term Eobulina. The gradational transition from 

 one form of aperture to the other, is seen in the varieties of the common Cristellaria calcar. 



252. Another set of varietal modifications of the Nodosarian type, arises out of the diver- 

 sities in the mode in which the segments are applied one to the other, and in the degree in which 

 the later segments extend themselves backwards so as to enclose the earlier. Thus in the 

 Glandidina, of D'Orbigny, not only is the anterior wall of each chamber prolonged forwards as in 

 Nodosaria, but the posterior portion of each segment is so far prolonged over the preceding as 

 to conceal a large part of it (Plate XII, fig. 5). This prolongation is carried to a much greater 



extent in the well-marked form known under the designation Frondi- 

 I'lG. XXXI. cularia ; of which the shell is extremely compressed, and of which the 



segments are so prolonged backwards along the two margins, that the 

 chambers have a sagittate form, the aperture being at the point of each 

 (Fig. XXXI). The extent of these prolongations, however, varies 

 considerably ; for sometimes the ahe. of the newer segments meet 

 beyond the primordial cell, so as completely to enclose it (ex, 

 fig. 50), whilst in other instances the posterior prolongation is not 

 greater than in many Glandulina (ex, fig. 51). When the early 

 growth of this frondicularian variety takes place along a spiral 

 instead of a rectilineal axis, (Plate XII, fig. 6), it is referred by 

 D'Orbigny to the genus Flabellina. The relation of this form to 

 Frondicidaria is precisely that of Cristellaria to Nodosaria .- that 

 general physiognomy of the two which depends upon the peculiarity of their later growths, 



