4o6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Metatoceras cavaiiformis Hyatt, and some of the lines 

 or sutures made in the external surface of the cast by 

 the intersections of the partitions or septa that cut 

 up the coiled tube of the living shell into air cham- 

 bers. Fig. ii6 shows a broken specimen of the same 

 species, but with the outer and older whorls in large 

 part removed. The innermost septum near the center 

 of the coil was built across the interior after the animal 

 had constructed the hollow apex or point. It then 

 moved along, adding to the external wall of the tube, 



Fig. 115. 



which has been destroyed and removed from this cast, 

 and built the' second septum, and so on until it reached 

 the tenth septum. By some freak of fossilization a 

 number of the septa beyond this have been destroyed, 

 so that if we were to remove the fragment of the ex- 

 ternal whorl and take out the center which has just 

 been described, this would have the exact aspect of a 

 cast of a young shell with ten air chambers. ^ The 



IThe shaded area in the center, shaped like a large inverted comma, was 

 an open space in the living shell. This is almost invariably filled by the 



