The large head, tapering body, and fanlike pectoral fins of the sea robin somewhat suggest a sculpin. But the robin is distinguished from all the sculpins by the incasement of its entire head in bony plates; by its smaller mouth; by the flat depressed dorsal profile of its snout; by its large ventral fins; and by the fact that the three lower rays of each of its pectoral fins are separate from the rest of the fin and modified into three independent feelers with slightly dilated tips, a very noticeable and distinctive feature. Furthermore, the front margin of the upper jaw is concave in outline when viewed from above, not convex as it is in most other fishes, which gives the nose of the robin a characteristic aspect. The head plates are rough and there is one sharp spine on each cheek at the angle of the gill cover; two short spines over each eye pointing backward; a spine on either side of the neck; and one on each shoulder above the base of the pectoral fin. The spiny and soft-rayed portions of the dorsal fin are separate, but they are so close together at their bases that they are almost in contact. The spiny dorsal has 10 spines, is rounded in outline, and higher than the soft dorsal (13 rays); but the soft dorsal is considerably longer than the spiny dorsal.
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