CRISTELLARIA. 73 



Gristellarise have such narrow and much-curved chambers that they are separable 

 as G. vortex ; and there are other extreme conditions of variable features 

 which have been conveniently adopted as grounds for separate names. Some 

 Gristellarise are not discoidal, but have become oval, and more or less oblong by 

 the growth of segments that leave the spiral arrangement, and follow a nearly 

 direct line, with their inner edges reaching down (backwards) to touch the spiral 

 part of the shell ; and they thus become elongate and either flattened, as 

 Gristellaria crepidula (Fichtel and Moll), or thick and subtriangular in section, as 

 C. Italica (Defrance). Others keep their later chambers quite free of the spire, 

 like G. sihharcuatula (Walker and Jacob), and thus pass into Marginulina. 



There are other less marked distinctions of subordinate importance, depending 

 iipon surface-ornamentation, the thickening of the central portion of the shell into 

 an umbo, and the septal lines into ribs, or other similar characters, due to 

 exogenous shell-growth. The shape of the pseudopodial or stoloniferous orifice 

 is, as in other Foraminifera, somewhat variable. In G. cnltrata it is often 

 triangular; and upon specimens having this peculiarity M. d'Orbigny founded his 

 genus Bobulina. Professor Williamson, Dr. Carpenter, and others have shown 

 how untenable such a distinction really is. 



Gristellaria calcar (Linne) serves as a central type for this sub-group of 

 Nodosarina. We have already indicated the interchangeableness of the 

 Gristellarise with others of this great generic group. Without entering further 

 upon the intricate polymorphism of Gristellaria, we proceed to describe our 

 specimens from the Crag. 



We begin with G. cultrata (Montfort) ; and ignoring the non-essential 

 diflFerences in carination, limbation, thickening of umbones, and relative size and 

 gibbosity of the chambers, we have to draw up a very long list of synonyms for 

 the little Gristellaria before us, which belongs to the form known as G. cultrata. 

 Indeed, zoologically it is very difficult to separate G. cultrata from G. rotnlatn and 

 G. Italica on one hand, or from G. calcar and G. cassis on the other, whilst the 

 distinctions of ornament are found to fade away one into another. With the 

 Gristellarise, as with other Foraminiferal groups, we have to deal with the 

 variability of individuals ; and for convenience of grouping and reference we must 

 artificially define nominal species, varieties, and even sub-varieties. We confine 

 ourselves, therefore, to the smooth, orbicular, keeled Gristellarise, with a modicum 

 of septal overgrowth and of umbonal thickening, as typical of G. cultrata. 



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