I9I2. No. 8. THE QUATERNARY SECTION OF KILEBU. I3 
cases the number of pronounced ribs may rise to twenty. It may be 
of a more compressed form or appear as a more arched variety. In some 
few specimens the peripheral edge is somewhat thickened and incurvate, 
so as to give them a crebricostate shape. A systematic measurement 
was elaborated in order to obtain some biometrical values that might 
be of some importance as to the extended knowledge of biological 
conditions during the time of deposition. 
Length Number Per cent 
IO—20 mm. 2 0,3 
20—30 I I3 I,9 
30—40 » 24 »5 
40—50 » 43 6,3 
50—60 » 81 11,9 
60—70 » 244 36,0 
70—80 » 230 33:9 
80—90 » 39 57 
91 &97 » 2 0,3 
In order to get a static expression of the law of variation as to the 
development of Pecten in this case, we might easily, from the above 
numbers, draw a curve on a paper divided into squares. 
Mytilus edulis L. occurred in a number of different varieties. The 
most common length of this mussel was 45—60 mm., but, specimens 
were even measured of length 80—9o mm. In very small specimens the 
difference between M. edulis and modiolus might seem to be rather 
dubious, but, even in that case, M. edulis is quite easily separated through 
the three prominent teeth and the one subordinate tooth of each shell. 
The occurrence of M. edulis in the different layers has been described 
above and we are here only to add some few remarks on the different 
varieties. Beside the most common forma typica we also find numerous 
specimens of either a more straight shape or of a rather curved variety. 
Between those prominent types, however, there is a multitude of inter- 
mediate ones. As a rule, this species has a plain blue or somewhat 
violet colour. Numerous specimens are, however, streaked in a very fine 
manner longitudinally, either with pretty blue or violet streaks on a 
yellow ground, or with vellow streaks on a violet ground. 
Mytilus modiolus L. also occurs in different varieties being either 
of a more slender shape or of a broader one. In the collection of glacial 
fossils from this locality at the university of Christiania, we find this 
species labelled as Mytilus umbilicatus Penn. and this nomenclature we 
