8 NILS ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. V. PROSOBRANCHIA. 1 DIOTOCARDIA. 
Specimens from southern Scandinavia (western coasts of Sweden and Norway) 
differ, as may be seen, in their essential characters from the forms of the Arctic 
districts ; in the first place they attain a considerable size, and besides this, are dif- 
ferent in some other respects, namely as to height and as to the position of the apex. 
The height in large specimens is, comparatively speaking, very small, and the apex 
has a more central position than is otherwise the case, but specimens from northern 
Norway, though in other respects like those of Spitzbergen, manifest the same char- 
acteristies. On Spitzbergen and Greenland the variation is uniform, but a larger 
size seems to be more common in the first-named district. 
Instead of such variations being expressed graphically, they may be indicated 
tabularly by stating the limits of variation-extent. It is necessary to determine these 
limits for different stages of growth; on the basis of these statements the curve can 
easily be drawn up in its entirety. Thus the curves of Puncturella noachina and Mar- 
garita groenlandica given above can be tabularly represented as on pag. 39 and 60—061. 
The examples advanced will be sufficient to show how the formal variation of 
shells may be expressed in such way as to provide some fixed points for judging of 
the mutual faunistic conditions of different districts. It is evident, that the charac- 
ters employed here might be augmented with one or two others, but I have consid- 
ered those included sufficient for determining the shell form in all essential re- 
spects. On these secondary characters depend, e. g. the width of the umbilicus, which 
seems to vary in direct proportion to the breadth of the shell; the apical angle and the 
convexity of whorls are characters of slighter importance than those above mentioned. 
In order to furnish a survey of structural variation, where this proves to be 
especially noteworthy, I have included, in certain cases, short accounts of the extreme 
forms of sculpture and the transitions between them. Instead of giving extensive 
accounts, I refer to the figures, which show the details in all their modifications 
better than any descriptions. 
In those cases where I have considered it particularly desirous for the sake of 
clearness, a short comprehensive survey of the variation of the radula has also been 
given. Here I again refer to the figures for details. 
The systematic arrangement of the prosobranchiate mollusces still remains unac- 
complished in many respect. I have here followed the system devised by BouvVIER, 
1887, which is at present accepted by the majority of writers on the subject, in the 
form in which it is given in LANG's »Lehrbuch der vergleichenden Anatomie, Mol- 
lusca», 1900. For the subdivision into families and genera as well as for the syno- 
nyms employed I have followed and adopted, with a few exceptions, PILSBRY's »Manual 
of Conchology» and the »List of British Marine Mollusca prepared by a Committee of 
the Conchological Society», second edition, 1902. 
The statements of length in the synopsis of species is given merely in order 
to show the size (medium to maximum) of the species, to serve as a help in their 
determination. Only the most important varieties are included in the systematic 
survey, others are mentioned in connection with the accounts of the distribution and 
the general variation. 
