What is the difference between skates and rays?

Skates and rays are very similar, with the major difference being in the way they reproduce.  Skates are egg laying animals, releasing their eggs in a hard rectangular case that is sometimes called a mermaid's purse, while rays give birth to live young.  Another difference between skates and rays is that skates have a prominent dorsal fin while the dorsal fin on rays is reduced or absent.  There are a few other minor differences between skates and rays; rays are generally larger than skates and they generally have a tail like a whip that has one or two stinging spines on the end.  Skates have fleshier tales and no spines.  Another difference is in the teeth of these two animals; skates tend to have small sharp teeth while rays have plate-like teeth.