The subclass Holocephali ("complete heads") is a taxon of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primitive forms, and the only surviving group in the subclass is the order Chimaeriformes. This group includes the rat fishes in the genus Chimaera, and the elephant fishes in the genus Callorhinchus. These fishes move by using sweeping movements of their large pectoral fins.

Holocephali. Retrieved May, 08 2021, from en.wikipedia.org/wiki/Holocephali.

The Chimaeridae, or short-nosed chimaeras, are a family of cartilaginous fish.

They resemble other chimaeras in general form and habits, but have short, rounded snouts, without the modifications found in related families. Many species have long, tapering tails, giving them an alternative name of ratfish. Shortnose chimaeras have a venomous spine on their backs, which is sufficiently dangerous to injure humans. They are found in temperate and tropical marine waters worldwide. Most species are restricted to depths below 200 m (660 ft), but a few, notably the spotted ratfish and rabbit fish, can locally be found at relatively shallow depths. They range from 38 to 150 cm (1. 25 to 4. 92 ft) in maximum total length, depending on species.

Chimaeridae. Retrieved May, 08 2021, from en.wikipedia.org/wiki/Chimaeridae.

The spotted ratfish (Hydrolagus colliei) is a chimaera found in the north-eastern Pacific Ocean. Often seen by divers at night in the Pacific Northwest, this cartilaginous fish gets its characteristic name from a pointed rat-like tail. The ratfish lays leathery egg cases on the bottom of muddy or sandy areas, which are often mistaken by divers as something inanimate. While mainly a deep-water species, it occurs at shallower depths in the northern part of its range. The generic name, Hydrolagus, comes from the Greek words ὕδωρ, meaning water, and λαγώς/λαγῶς, meaning hare, and the specific name honors Alexander Collie, who was a ship surgeon and early naturalist. The spotted ratfish is common in much of its range, not typically eaten by humans, and is not commercially caught.

Hydrolagus colliei. Retrieved May, 08 2021, from en.wikipedia.org/wiki/Spotted_ratfish.