Biology

Cephalaspis lyelli was a small fish, 20–30cm long with the following characteristics:

the head-shield is large - about a third of the length of the body
the shield is nearly as wide as it is long
the cornua are well developed and project backwards and outwards
the orbits are small and roughly in the middle of the shield - in the type specimen the head shield is about 8cm long, and 7.8cm wide
there is a small - 2cm - dorsal spine at the back of the shield, and the cornua are 2.8cm long
the inner margin of the cornua are smooth, and do not possess tooth-like structures called denticles, often seen in other genera
Cephalaspis lyelli has three openings in the skull ('fields') that are filled with small scales
the central dorsal field, just behind the eyes, is long, straight sided and narrow, 2.2cm long, and 0.7cm wide, at the widest point
the lateral fields near the edge of the shield extend onto the cornua, and are relatively broad