C.—GEOLOGY. 101 
which as at present used does not define a species but an important 
species-group, the earliest members of which occurring low down in the 
Ordovician are certainly markediy different even as regards the 
external ribbing of the shell from those occurring at the base of the 
Silurian, though all have been included in the same diagnosis. Up to 
the present all we can do in naming such a fossil is to term it, in despair, 
Dalmanella of elegantula type. 
This work can and must be tackled group by group; it will demand 
an amount of careful field-collecting, in the first place, of specimens 
showing internal as well as external characters, for these last are by 
no means to be neglected, since they often reflect changes in internal 
characters, though they do not do so invariably ; hence it will be neces- 
sary to distinguish between those possessing different internal and 
external characters and those which differing in their internal 
characters yet may have the same external characters. 
Field paleontology, when it has a definite aim of this sort in view, 
becomes a fascinating and absorbing study, and a fresh zest is given to 
the somewhat monotonous task of mere fossil-collecting. 
Kiaer, in his classic memoir on the Silurian Rocks of the Christiania 
Basin," has indicated to us how this work may be carried on. He was 
fortunate in that the rocks in the area where he did his work are but 
slightly inclined and are affected only by faulting and not by folding, 
so that there can be no doubt as to the order of succession of the various 
beds. To a large extent Kiaer has applied the principles of evolutional 
paleontology with great success ; he notes the appearance of early muta- 
tions and their gradual evolution at successive horizons up to and beyond 
the development of the typical form. Thus he utilises the evolution of 
the septum in the Pentamerids of the species group of P. oblongus; he 
notes how this septum is short in Barrandella undata, the earliest of the 
true Pentamerids, and shows how this gives place upward to another 
mutation, P. borealis, with a septum which, though rather longer, is 
nevertheless shorter than that of P. oblongus s.s., which is next 
developed. At a still definitely higher horizon is found P. gotlandicus, 
probably to be regarded as a late mutation of P. oblongus, in which 
the septum is still further developed. 
Having arranged these Pentamerids in order, Kiaer is able to throw 
light on the development and relation of the Stricklandinias, among 
which there has been and stili is much confusion in this country. He 
shows that Stricklandinia lens makes its appearance in the Christiania 
Basin with the borealis mutation of P. oblongus, and is followed at a 
slightly higher horizon by a mutation of its own, whereas S. lirata does 
not occur till the horizon of the galeatus mutation. 
For purposes of correlation, however, Kiaer notes the position of 
the beds containing the fossils in relation to the deeper-water Graptolite 
Shales. Thus, for example, beneath his zone of Barrandella undata he 
recognises the zone of Cl. normalis, the equivalent of our British zone 
of Diplog. acwminatus, and some little way above his zone of Penta- 
merus oblongus he notes the graptolite zone of Cyrtog. Murchisoni, 
141908, Kiaer, J. ‘Das Obersilur im Kristianiagebiet.’ 
