PRINCIPLES OF CLASSIFICATION. 



59 



and the septal plane s.p. is here extended on either side into the " alar prolongations " a. p., 

 a. p., until its angles h, h reach the umbilicus or centre of the spire. Now a difference of 

 this kind not unfrequently presents itself, • 



not only between different individuals Fig. XII. 



whose specific identity is demonstrated by 

 the gradational series that connects them, 

 but also between the different parts of one 

 and the same individual ; the latest whorl 

 often disengaging itself more or less com- 

 pletely, whilst each of the earlier whorls 

 was successively invested by that which 

 succeeded it. Hence it is obvious that no 

 such difference can be justly regarded as 

 a basis for specific distinction, until it shall 

 have been shown to be both constant in 

 its occurrence and uniform in its degree ; 

 the presumption being decidedly in favour 

 of its variability, until the contrary shall 



have been established. It is in the genus Nummulites that the peculiarities in the 

 disposition of the alar prolowjations of the chambers, and of their intervening septa, seem 

 to have the greatest importance as differential characters, and have been most used for the 

 discrimination of species; but we shall find reason to question whether even there such 

 peculiarities have the uniformity and definiteness which are required to justify such an 

 employment of them. 



72. Of surface-marUnfj there are two principal kinds, which are for the most part related 

 to the " porcellanous " and the " hyaline " types of shell-structure respectively. The surface 

 of the porcellanous shells, as already stated (t 57), is not unfrequently marked by striations 

 or by pittings, more or less conspicuous and regular in their arrangement ; and the value to 

 be attached to these must depend entirely upon the degree of constancy with which they present 

 themselves in each particular type. Thus in Vertehralina we shall find that the presence of 

 coarse striations, passing transversely between the septal bands (Plate V, figs. 17—25), is so far 

 constant in the first-formed portion of the shell (though very commonly wanting in the later 

 segments) as to afford important evidence in the determination of forms that might otherwise 

 be doubtful. So, again, the more delicate striation of PeneropUs (Plate VII, figs. 16, 18, 20) 

 is so constant and characteristic, that the exact similarity it presents in the " dendritine " and 

 " spiroline" forms (Plate VII, figs. 1,4, 12, 13, 21) becomes an important element in the deter- 

 mination of their merely varietal nature ; even its occasional obsolescence (figs. 2, 3) strengthen- 

 ing rather than weakening this conclusion, such obsolescence occurring after the same manner 

 in all these types. In MiUola, on the other hand, nothing can be less constant than the sculpture 

 which so remarkably distinguishes certain individuals (Plate VI, figs. 3, .5, 13, 14) as apparently 

 to justify their being ranked as well-characterised species ; for wherever a sufficient number of 

 individuals thus distinguished is brought together, there will be found some in which it is far 

 less conspicuous than usual, and others in which it is wholly wanting either on some part of 



