76 FAMILY MILIOLIDA. 



whole Miliolinc series is so simple, that it may be readily apprehended from a survey of the 

 external features of its fundamental type. Yet these features are liable to be so completely 

 masked, as altogether to lose their chai-acteristic aspect ; and since there hence arises so com- 

 plete a want of agreement between external characters and internal structure, as leads to the 

 entire misapprehension of the latter if too exclusive confidence be placed in the former, 

 it seems desirable to study both together in such a manner as to bring out their true relations. 

 And in this way we shall come to perceive the fundamental unity which prevails through a 

 range of varietal modifications so wide, as at first sight to appear to justify the creation of 

 an almost unlimited number of genera and species. 



104. The fundamental " idea" of a MiJiola is best displayed in such a form as that 

 represented in Plate YI, fig. 1, which shows but little departure from the continuous spiral of 

 Cornusjjir'i ; the diifcrence consisting chiefly in this, — that each turn of the spiral is interrupted 

 at two opposite points by a constriction followed by an enlargement : and that, as the inter- 

 ruptions in successive turns are always at the extremities of the same diameter, the whole 

 spire is made up of a series of half-turns arranged symmetrically on its two sides. Each of 

 these half-turns is larger than that Avhich preceded it, but its dimensions scarcely change 

 between its two ends; so that the increase in the diameter of the tube is not gradual, but 

 takes place at successive intervals, each chamber being not only longer than its predecessor 

 on the opposite side, but also larger in sectional area. The form of that area departs from 

 the circular, in consequence of the tendency which each turn of the spire has to extend itself 

 in some degree over the preceding. Although this extension is but slight in the example we 



. are considering, it is enough to give a concave instead of a convex border to the inner 

 wall of the chamber; which inner wall is, in fact, nothing else than the outer wall of the pre- 

 ceding turn, the shelly tube of the new chamber being there incomplete (^ 62). The 

 aperture of the last chamber is somewhat constricted : and it is further encroached-on bv 

 a tongue or tooth-like projection of shell substance from its inner margin (figs. 16 — 18) ; this 

 pi'ojection, which may be conveniently termed the " valve," varies greatly in size and form, 

 but when most developed, it converts what would otherwise have been a free nearly circular 

 passage into a comparative!}' narrow cresccntic slit. A similar " valve," whicli may be regarded 

 as a rudimentar}' septum (^ 109), exists at each of the constrictions that marks the division 

 of the chambers. 



105. It is comparatively rare, however, to find so slight a departure from the regular 

 spiral plan, as is shown in specimens of the kind just described. For in by far the larger 

 number of cases, the diameter at the two extremities of which the septal constrictions occur 

 is more or less elongated ; and this elongation may proceed so far as to give it twice the 

 length of the transverse diameter (fig. 2), thus substituting for the spirality of the original 

 type an apparently Uhiicral arrangement, which has caused D'Orbigny and his followers to 

 define the Miliolinc group as if the shell were composed of " chambers clustered ='■ on two, 

 three, four, or five faces of a common axis. 



* For the word pelototmees, \Tliich is intended to indicate the kind of aggregation produced by 

 ■winding a ball of cotton, there is no proper equivalent iu our language. 



