Quick Aquarium Lesson

I was asked at one point what the difference between a skate and a ray was. Here's an explanation I found from Sharks, Skates, and Rays of the Carolinas by Frank J. Schwartz:

Skates and rays are diamond shaped, flattened dorso-ventrally, and except for mantas (Manta birostris), butterfly rays (Gymnura sp.), southern stingrays (Dasyatis americana), and roughtail stingrays (Dasyatis centroura), rarely exceed 1-meter wing disk widths (DW). Skates can be further distinguished from stingrays as they possess two dorsal fins on the tail, have a distinct caudal fin, possess thorns on the tail, and lack poisonous serrated spines on the tail. Stingrays possess one or no dorsal fins, and their whiplike tails are usually armed with one or more serrated spines that possess venom-producing goblet cells located along the length of ventral grooves of the spine. Tail length may vary from equal to or several times longer than the body length (roughtail stingray). All stingrays have 5 gill slits, except an African species that has 6 (Hexatrygon bickelli) (Heemstra and Smith 1980).