Both Selachains and batoids are considered Elasmobranchs, a subclass of the Chondrochtyes. Both of these fishes have cartilaginous skeletons and plated gills. Additionally both have placoid scales, which are similar in strucutre to teeth, giving them a rough sand-paper like skin. Since these groups are a subclass of elasmobranchs, they also have polydenton dentitions. That is to say there are many rows of replacable teeth.
Selachians include sharks. All sharks have 5-7 gill slits on the side of their head. These also have no swimbladder. Rather they rely on their large pectoral fins and oily livers to maintain bounency. Some sharks have the ability to osmoregulate, and can occupy a large range of ecosystems, marine or otherwise.
Batoids are fish such as rays and skates. These are dorsal ventrally flatened fish with pectoral fins fused to the side of their heads. Rather than having their gills on the side of their head, their gills are on located ventrally. Rays are viviporous and give birth to live young, while skaes are oviparous and lay eggs.
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