File:The Quarterly journal of the Geological Society of London (13206038635).jpg

VON BUCK ON BEAR ISLAND. 55
Island, although in these islands the characteristic petrifactions are
enveloped in sandstone and not in limestone ; and in this manner the
Scandinavian peninsula as a centre is enclosed on the northern, the
eastern, and the southern sides ; — a nucleus round which a whole
system of formations are grouped, and which has been the seat of
mighty revolutions on the surface of the earth. That the southern
limit of the Productus formation has been advanced so far to the south
by the knowledge of the stratification of Bear Island is an important
gain to geology, for which we are indebted to Keilhau's researches.
Ventral valve. Dorsal valve.
Spirifer Keilhavii (fig. 3) belongs to that division of Spirifers
(the winged, Alati) in which the hinge-border is not curved, and is as
wide, or even wider, than the greatest breadth of the shell ; the sides
descend perpendicularly from the ends of the hinge-border, and unite
with the front by a gentle curve. Thick broad ribs (Falten) cover
the shell, and only six or eight on each side. These ribs are again
divided, not only once, but each rib has usually at most two smaller
ones on the sides. They are only simple towards the beak. These
broad divided ribs particularly distinguish this Spirifer from others
similar to it. In the sinus only the finer ribs are observed, and no
larger ones. The beak is strongly curved over, and the area is con-
sequently low and itself somewhat arched. The ventral valve is only
moderately arched, indeed far less so than is usual in similar forms ;
the dorsal valve rises much higher. This dorsal valve is thickly
covered with those siliceous groups and rings (Kieselsystemen und
Kieselringen) which are always observed where shells become silicified.
Here also the silicification takes place below the calcareous shell, which
is pushed off and destroyed by the siliceous rings.
This Spirifer is nearly two inches broad, and almost as long.
Length, 100; breadth, 111; thickness, 48; breadth to length of
sinus = 40 : 100.
The division or dichotomy of the ribs and striae is by no means so