100 STUDIES ON THE ELASMOBRANCH SKELETON, 



structure of the pectoral fin presenting in any way an approxi- 

 mation to the Rays. 



The pelvic arch is a nearly straight bar, slightly arched forwards 

 in the middle, with short, broad, anteriorly directed processes at 

 either end. There is a short pre-axial ray or propterygium, with 

 which three short rays are connected ; there is no representative 

 of a mesopterygium, but none of the rays articulate directly with 

 the pelvic arch, the long narrow metapterygium supporting all the 

 rest. 



Unpaired Fins. 



The unpaired fins present broad plate-like basal cartilages 

 closely united with the vertebra?. 



The verteba? of Pristio2)horus present solid, deeply biconcave 

 centi'a, which on a mesial transverse vertical section exhibit, as in 

 the case of Selache, two pairs of cartilaginous tracts passing from 

 near the cenfo-e to the supero-lateral and infero-lateral parts of the 

 vetebrse. 



TRYGON PASTINACA. 



Plate II., Figs. 10-12. 



Vertebral Column. (Plate II., figs. 10 and 11.) 



The anterior vertebral plate presents a very prominent, con- 

 tinuous, spinous ridge. In the hinder half of the plate the 

 transverse processes are likewise developed on each side into a 

 wide thin lamina perforated at its base by 9 or 10 irregular 

 apertures. In front this lamina is developed into a free flap. 

 Behind it is continuous with a plate of cartilage which stretches 

 horizontally across the middle line, becoming synchondrosed with 

 the spinous ridge. The horizontal plate corresponds to the 

 horizontal bar found in a corresponding position in Hypnos, but, 

 differs from the latter in being closely united with the spinous 

 ridge. It is connected externally, as in Hypnos also, with 

 the pectoral arch, the connection being effected by means 



