Usage: capitalized
: a class of lowly vertebrates formerly an order or subclass of Pisces but now segregated with certain extinct related forms in a superclass Agnatha that are elongated creatures resembling eels with a large jawless sucking mouth, no limbs or paired fins, a wholly cartilaginous skeleton with persistent notochord, and 6 to 14 pairs of gill pouches supported by a system of cartilaginous rods rudimentary in the hagfishes and that include the lampreys and the hagfishes
Useful english dictionary. 2012.