This medium to large acanthodian had very stout, striated spines. The spines were deeply inserted in the body. This meant they were not easy to bend, and this fish was, therefore not an easy meal for any predator. Diplacanthus tenuistriatus is extremely rare in the Sandwick Fish Bed. Loose spines and scales can be found at other localities on Orkney, but only very seldomly. The specimen shown is missing the head and is from Achanarras.
Between the pectoral spines are two small spines that are positioned flat along the ventral side of the fish. These spines are called the anterior intermediate spines. A bit further along the ventral side towards the pelvic spines are another two spines, which are called the posterior intermediate spines. The pectoral girdle can be roughly compared with Diplacanthus crassisimus. Two isolated half pectoral girdles are figured here.