The cranial osteology and the dentition of Tristichopterus have been hitherto entirely unknown, and we were but imperfectly acquainted with the structure of the pectoral fins. Consequently great doubts have prevailed with regard to its affinities, though it was supposed to be allied in many respects to Dipterus. A suite of specimens from John O'Groat's, in the Edinburgh Museum of Science, collected by Mr Peach, the original discoverer of the fish, subsequent to the publication of Sir Philip Egerton's description, throws great light on the previously unknown points of its structure, as well as on its affinities.