BAL^NOPTERA. 



47 



18. Chevrons, six of the cinterior, from the same Finner. 



In the fuetus twenty-one chevrons were counted : 

 one was almost exclusively cartilage ; four were 

 mostly cartilage ; sixteen had a larger proportion 

 of bone. They were attached to the ventral surface 

 of the intervertebral discs. The chevron cartilage 

 was enclosed in its own perichondrium, and was not 

 continuous with the cartilage of the spine. A centre 

 of ossification had formed in the cartilage plate on 

 each side and extended into the mesial plate to form 

 the ventral spine, except that in the last six the lateral 

 plates lay in distinct pericliondrial sheaths and did 

 not meet mesially. The chevrons extended as far as 

 the fourth vertebra in front of the tip of the tail, and 

 the last was only h inch by | inch in diameter. The 

 1st was smaller than the 2nd, each plate of which was 



2 inches long by IJ inch wide at its attachment 

 to the intervertebral disc. Behind the 2nd the 

 chevrons for some distance increased in size, and 

 then diminished at the hinder end of the series. 



19. Rib, last left, and vertebral end of last right rib ; left 3 feet 



3 inches long, slightly curved ; chord of arc 3 feet 

 1| inch. 



20. Thoracic inlet bounded by the Sternum, a cruciform 



flattened bone with a short posterior limb ; transverse 

 diameter 26| inches, antero-posterior diameter 17-J 

 inches ; articulates with 1st pair of Ribs, which also 

 articulate with transverse processes of 1st dorsal 



