Cosmoid Scale

a scale of primitive Crossopterygii and Dipnoi fishes whose external surface is formed of a layer of cosmine (hence the name)—a continuous “parquet” pattern of tightly joined cutaneous teeth. The cosmoid scale is covered on top by a hard enamel-like dentin, which imparts its characteristic sheen. The cosmine is lined with a layer of spongy bone; at the base of the cosmoid scale lies a large layer of lamellar bone— isopedin. In the evolution of the Crossopterygii and Dipnoi, the external and spongy layers of the cosmoid scale gradually were reduced. A few isolated nodules of dentin have been preserved on the surface of the scale of the modern crossopterygian Latimeria.