198 W. HAKOLD LEIGH-SHARPE 



than indicate a likeness to the skates in general "as regards the 

 claspers, which are well developed, suggesting a male of mature 

 age. 



SUMMARY 



The conclusions drawn from a chronological survey of fossil 

 forms are: 



1. The oldest elasmobranchs (Cladoselache) are without 

 claspers. It is almost certain they are without clasper siphons 

 and clasper glands also. 



2. The next fossils have a direct type of clasper. Possibly 

 clasper siphons were not yet evolved or were present only in a 

 rudimentary form. 



3. Subsequently the scroll type of claspers appeared, sug- 

 gestive of the Scylliidae. Probably these were accompanied by 

 a clasper siphon. 



4. The Lamnidae are geologically more recent than the 

 Scylliidae, and these have progressed a stage further and evolved 

 a clasper gland, at any rate in recent forms. 



5. Later the skates arrived and resemble those of recent times. 



