60 OF THE FORAMINIFERA GENERALLY : 



the external surface or at some period of growth ; so that, a continuous gradation being thus 

 estabhshed between the most regularly sculptured and the perfectly smooth forms, it becomes 

 obvious that no valid specific distinction can be erected on such a basis in this type. 



73. Among the "hyaline" shells, on the other hand, variety is given to the surface 

 marking chiefly by the interposition of bands or spots of non-tubular substance in the 

 midst of the tubular ; such portions being distinguished by their vitreous lustre from the 

 general surface, even when they do not project above it. Most commonly, however, they 

 are raised into ridges or tubercles ; and these are sometimes arranged with great regularity, 

 whilst in other instances they are extremely variable. Generally speaking, we find that when 

 either continuous bands or rows of spots of non-tubular substance repeat themselves with 

 anything like regularity in a direction trans vers/' to that of growth, these mark the position of 

 subjacent septa ; and, by the elevation of these septal bands, we have septal ridges or rows of 

 tubercles, such as are often strongly marked in the " cristellarian" type of Nodosaria (ex, PI. ii, 

 fig. 54), in Oppi-cuUna (Plate XVII, fig. 1), and in Cycloclypeiis (Plate XIX, fig. 2), But, between 

 these septal ridges, we often find a multitude of tubercular elevations ; sometimes arranged in 

 regular transverse rows, as in certain varieties of OpcrcuUna ; more commonly, however, with- 

 out any such symmetry, as in many Rotalia and TlanorbtdincB . These, instead of the vitreous 

 lustre, sometimes exhibit an opaque porcellanous whiteness (Plate XIII, fig. 15). In cases in 

 which the original walls of the shell are overlaid by subsecpient deposits, we very commonly find 

 that the size and prominence of these tubercles increase with every addition to its thickness ; so 

 that in section they present the appearance of cones whose base is at the surface of the shell, 

 whilst their apex points to its interior. This is often strikingly displayed in the umbilical 

 region of Opcrculhia (Plate XVII, fig. 1), and in the central regions of CydocJypeus (Plate XIX, 

 figs. 2, 5) and of Orbituides (Plate XX, fig. 2). The variability of any such ridges or tubercles, 

 however, is such as altogether to destroy their value as specific characters ; individuals in 

 which they present themselves under so pronounced and peculiar an aspect as to seem defi- 

 nitely differentiated by their presence from the ordinarj' type, being found to be connected 

 with it by a continuously gradational series ; whilst even on diff"erent parts of the very same 

 shell, the size, disposition, and aspect of the tubercles are found to vary so much as to render 

 exactness of definition altogether impossible. 



m 



74. Another kind of surface-marking in the "hyaline " series is given by ridges which 

 project from the shell, not transversely, but lonyHudinally, that is, in the direction of growth ; 

 these are very common in the genus Lagena (ex, PI. i, figs. 8-14), and in the protean forms of the 

 Nodosarian type (ex, PI. ii, figs. 36-48) ; but they are of no more value to the systematist than 

 those already noticed, since they vary greatly in the degree in which tliey are developed in 

 different individuals, and are frequently wanting on portions of shells which elsewhere present 

 them very strongly marked. The most remarkable modification of this kind of surface-marking 

 with which I am acquainted is the hexagonal areolation presented by certain " entosolenian '' 

 varieties of Layena (see ex, PI. i, figs. 29-32) ; ,this, however, is not more constant than the 

 similar areolation of certain varieties of Miliola, although it would at first seem to have more 

 value as a differential character on account of the difference of texture between the shell 

 substance of the ridges and that which forms the general surface. One other variety of 



