174 FAMILY GLOBIGERINIDA. 



modifications it becomes biserial. These two generic types differ essentially, however, like 

 TJvigprina and FoJymorphlna, in the setting-on of the chambers, and in the form of the 

 aperture. In Chri/mUdlna and CuneoJina, whose plan of growth is Textularine, the large, 

 single aperture is replaced by a multiplication of pores. And in Cunsidtdma, of which the 

 aperture is Bulimine, and the chambers alternate biserially, the elongated axis is itself coiled 

 into a spiral, affording the only real example of that complex plan of growth on which 

 D'Orbigny founded his order Enallostegues.* As the Nodosarine type culminates in 

 CristeJlaria, so may we consider the Textularine as culminating in Cussidullna ; and the 

 latter of these genera, like the former, is altogether destitute of those peculiar features which 

 characterise what may be unmistakeably recognised as the most elevated forms of the 

 family Glohigerinida. 



276. To such forms we can only ascend by returning again to those simpler derivations 

 from the Globigerine and SpiriUine types, which constitute the lowest forms of the Bofaline 

 series : this consists of a group of genera whose typical character consists in the arrange- 

 ment of the chambers in a turbinoid spiral, and in their communication with each other by 

 an aperture (usually more or less crescentic in form) situated at its inner and lower 

 margin. Now the simplest forms of this Rotaline spire present a close parallel to the 

 Textularine series in the manner in which each segment applies itself to the preceding ; but 

 the same type may be traced upwards, through successive grades of development, until we 

 arrive, in the highest form of Rotalia Beccarii, at a condition scarcely inferior to that of Ojier- 

 culina in I'egard to the differentiation and duplication of the septal laminae, the system of intra- 

 septal canals, and the supplemental skeleton. In the genus Calcarina we find an exogenous 

 develoj^ment of the supplemental skeleton, and of the portion of the canal-system provided for 

 its nutrition, which is unparalleled elsewhere; but it is not a little singular that this modification 

 is related rather to the higher than to the lower forms of the Rotaline series ; the septal 

 laminae being for the most part single, so that the intraseptal portion of the canal-system is 

 but little developed. It is further worthy of note in this genus, that the narrow, crescentic, 

 apertural slit is divided by cross-bars into separate circular pores. 



277. The greatest departure from the normal development of the Rotaline type is shown 

 in the entire cessation of the regular plan of increase, observable in all but the earliest stage 

 of growth of certain forms, which, in the structure of their individual segments, are entirely 

 conformable to the Globigerine type. The essentially Rotaline character of these organisms 

 is marked by the tui-binoid spiral in which they usually commence ; whilst the forms they pre- 

 sent in their fully developed condition are so far from suggesting their real relationship, as to 

 seem in many instances to claim for them a completely separate place in the series. By the 



* The genera Asterigcrina and Heterostegina, whicli were placed by D'Orbigny in this order, 

 have no claim whatever to be thus differentiated from their congeners ; and although in Amphistegina 

 there is often a want of equality between the alar extensions of the chambers on the two lateral sur- 

 faces, with a separation of the umbilical extremities of those extensions into secondary chamberlets, 

 there is no really biserial alteration. 



