REPORT OH THE FORAMINIFERA. 
355 
and position, that of Tritaxia terminal and rounded, and that of Chrysalidina porous. 
Valvulina, in like manner, may be recognised by its aperture, which is an arched fissure 
partially closed by a valvular tongue ; but it also differs from the allied forms in the 
disposition of its segments as well as in other particulars. The genera Bigenerina, 
Spiroplecta, Gandryina, and Clavulina represent dimorphous types, closely related in one 
way or other to those already enumerated. Pavonina, though irregularly dimorphous in 
its mode of growth, stands somewhat apart, there being no corresponding monomorphous 
type to which it can be referred. The minute structure of the test in the Textularin^e 
displays greater variety than in almost any other group of Foraminifera of similar extent. 
In some species the shell-wall is thin, calcareous, transparent, and perforate, whilst in 
others it is coarsely arenaceous, and rough externally ; and, between these extremes, almost 
every variety of texture may be met with. Superficial ornament of any sort is of 
comparatively rare occurrence. 
In the Sub-family Bulimininee the typical plan of structure is that of a more or less 
elongated spire, with three or more segments in each convolution ; but this becomes 
simplified in the weaker modifications. The genus Bulimina has spiral test of the 
typical description, but in Virgulina the whorls are irregular and few-cliambered, and in 
Bolivina the segments take a definitely biserial arrangement. These forms have their 
dimorphous representative in the genus Bifarina. Plcurostomella differs from Boli- 
vina chiefly in the character of the orifice. The normal aperture of the Buliminijre 
is a comma-shaped slit or loop, set oblicpely on the inner face of the terminal segment; 
and the various forms which it assumes may generally be recognised as modifications of 
that typical condition. In all the recent Buliminesee the shell-wall is hyaline and 
perforate, and in certain species it is ornamented externally with longitudinal striae 
or costae, or with sharp spines. The texture of many of the larger fossil forms is 
subarenaceous, though the test retains its perforate character, and is seldom coarsely 
rugose externally. 
The Sub-family Cassidulinesee constitutes an essentially biserial group. The primary 
arrangement of the segments is that of Textidaria, or rather, having reference to 
the nature of the aperture, of Bolivina ; but in the typical condition the structure is 
complicated by the helicoid mode of growth, — the biserial line of segments being 
partially folded lengthwise, and then coiled upon itself from the primordial end. In 
the genus Cassidulina the test is either convoluted and involute throughout, or the 
later portion is projected in a straight or curved line. In Ehrenbergina the shell is 
unfolded and unrolled almost from the beginning, and so furnishes the connecting link 
between Cassidulina and the biserial Buliminiisee. In Orthoplecta the segments are 
combined in an irregularly alternating or spiral series, and form a cylindrical test without 
helicoid commencement. The aperture in the Cassiduliniisee resembles in all respects 
that of the Buliminiiee. The minute structure of the investment, whether in recent or 
