Astraspis

Astraspis ('star shield') is an extinct genus of primitive jawless fish from the Ordovician of Central North America including the Harding Sandstone of Colorado and Bighorn Mountains of Wyoming. It is also known from Bolivia. It is related to other Ordovician fishes, such as the South American Sacabambaspis, and the Australian Arandaspis.

Description
Nearly complete fossils suggest the living animals were about 200 mm in length. The body had a mobile tail covered with small protective plate-like scales of less than 1 mm and a forebody covered with plate-like scales larger than 2 mm. The specimen from North America (described by Sansom et al., 1997) is to have had relatively large, laterally-positioned eyes and a series of eight gill openings on each side. The specimen was generally oval in cross-section. The protective bony plates covering the animal were composed of aspidin (chemically similar to modern shark's teeth), covered by tubercles composed of dentine. It is from these tubercles (which are generally star-shaped) that the name 'Astraspis' (literally "star-shield") is derived.

Other sources
Michael J. Benton, Vertebrate Palaeontology, 3rd edition, 2005