Hemicyclaspis is an extinct genus of primitive jawless fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, closely related to Cephalaspis

Cephalaspis

Cephalaspis was a genus of armored goldfish-sized to trout-sized detritivorous fish that lived in freshwater streams and estuaries of early Devonian Western Europe...

, that lived in the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period in what is now Europe and North America.

A typical cephalaspi

Cephalaspidomorphi

Cephalaspidomorphs are a group of jawless fishes named for the cephalaspids, a group of osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids...

d, Hemicyclaspis had a heavily armored

Armoured warfare

Armoured warfare or tank warfare is the use of armoured fighting vehicles in modern warfare. It is a major component of modern methods of war....

, shovel-shaped headshield. It is thought to have been a better swimmer than most of its relatives because of its powerful tail, stabilizing dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

and the keel-shaped hydrodynamic edges of its head shield. Hemicyclaspis probably foraged the ocean floor for food.

Hemicyclaspis grew to a length of 5 inches (13 centimeters) and would most likely have fed on particles from the muddy sea bed.