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 

 characteristic and decisive among Spirifers as among Terebratulse ; 



