GENUS VALVULINA. 147 



on the convcxit}- of its last chamber a large circular aperture, to which is applied a circular 

 flap or "valve" of rather smaller diameter, so as to leave a fissure which would be completely 

 circular but for the tongue-like process which connects the valve with the umbilical margin of 

 the aperture. The shell may lose its trifacial compression, and assume an ordinary trochoid 

 or conical form, without any departure from the triserial arrangement of its chambers. This 

 form is sometimes extremely depressed, becoming so flat as to resemble a Rofa/ia (fig. 16); ' 

 and its aperture is narrowed to an irregular slit (fig. 23), the valve being reduced to a slightly 

 raised lip along its umbiUcal margin. Such a form has been described and figured by Prof. 

 Williamson (ex, p. 55, figs. 114, 115) under the name ILotalina Jmca. It is distinguished 

 from Rotalia not only by the arenaceous character of the shell, but also by the uniformity 

 with which each circuit of the spire is completed by three chambers, the last three being 

 always exposed at the base of the cone. This depressed conical form attains a larger size 

 than any other, and is the one most widely distributed. The cone varies considerably in its 

 proportions, but always exhibits at its apex a relatively large, globular, primordial chamber, 

 which is sometimes separated by a slight constriction from the chambers that succeed it. 

 The preceding is often accompanied by a more aberrant variety, in which the triserial 

 arrangement is altogether wanting ; and the several chambers of which the shell is composed, 

 failing to make even a single coil of the spire, form an obliquely semioval shell, having a 

 broad, flat, oblique apertural plane, with a very large aperture, and a valve of extraordinary 

 dimensions (figs. 21, 24). The crescentic fissure that surrounds the valve is crossed by 

 numerous processes of shell, which pass from the margin of the valve to that of the aperture, 

 and is thus converted into a series of separate passages ; and the valve being itself perforated 

 by large pores, the entire apertural surface has an almost cribriform aspect. 



221. Another series of varietal forms results from the substitution of a new plan of 

 growth for the original triserial arrangement, and the consequent entire departure in the last 

 formed portion of the shell from the triangular form which its earlier portion exhibits. Thus, 

 the proper " valvuline " may give place to a " bulimine '' succession of chambers, the spire 

 becoming rounded to the form of that of a Bulhnm, and each coil being made up of several 

 oblong, parallel segments (fig. 25). The characteristic form of valve in this variety is a tongue 

 or flap formed by a curved prolongation of the turgid lip of the aperture (fig. 19) ; but this 

 is frequently modified so as to give place to other forms, such as those represented in figs. 20, 2(). 

 In some specimens of this variety the early triangular growth is so completely masked, that 

 it is by the presence of the valve alone that they can be distinguished from the sandy forms 

 of Bulimina. — Again, the proper " valvuline" fonii may give place to an elongated, nail-like, or 

 " claviform " shape ; and it is on this modification, occurring not merely in Valvulina, Taut also 

 in Textularia and Uvigerina, that D'Orbigny has founded his genus Clavnlina. In tlie clavi- 

 form Valwlina we find the primitive triserial arrangement of the chambers replaced by a 

 uniserial, with little or no progressive enlargement of the chambers ; sometimes with a 

 marked distinction in external form, the uniserial portion being cylindrical, or nearly so 

 (fig. 17); but sometimes with a retention of the triangular shape, which is here due to the 

 triangular form of the chambers, the angles of which are strongly carinated (fig. 18). The 

 mouth in cither case is terminal, and is usually furnished with a regular valve (fig. 17, a). 

 A still more aberrant claviform variety is occasionally met with, in which the uniserial 



