Various types of trilobite exoskeletal sculpture are described and referred to a simple classification. Thin-section and surface examination of many such sculptural features shows them to be remnants of distinct cuticular organelles, which probably had a sensory function. The supposed sensory structures of well-preserved specimens of Phacops rana (Green) are described in detail and their presumed specific functions considered. Mapping sensilla distribution over the Phacops rana dorsal exoskeleton shows a distinctive pattern conveniently divisible into nine inferred sensory fields with the densest sensory concentrations on the head and margins of the dorsal shield.

Conclusions on the importance of these sensory fields in mediating the trilobite's behaviour are combined with evidence from general morphology, palaeocology and evolutionary trends to suggest that Phacops rana was an active benthonic animal with a dual trophic capability lending considerable flexibility in adaptive radiation.

The P. rana hypostomal suture, consisting of two short coplanar lateral branches, is described for the first time, indicating that the hypostoma had considerable freedom of movement, possibly connected with ingestion of bulky food.