Unquestionably the most original part of his scientific work, that wherein he added most to the sum of acquired knowledge, is to be found in his reconstruction of the extinct types of fishes which he discovered in the Old Red Sandstone. The merit of these labours can hardly be properly appreciated unless it be borne in mind that he came to the study of the subject with no preliminary biological training, save what he could pick up for himself from an examination of such denizens of the waters of the neighbouring firths as he could meet with. But after prolonged search he could find in these northern seas no living creatures, the structure of which afforded him any clue to that of the fossil fishes of Cromarty. Some men had concluded that the organisms were ancient turtles, others that they were crustaceans, or even aquatic beetles. He had the sagacity, however, to surmise that they were probably all fishes, and he enjoyed the satisfaction afterwards of learning that Agassiz pronounced even the most bizarre amongst them to belong to that great division of the animal kingdom. He was guided by his own intuitive conception of what must have been the plan on which these long vanished types of organic structure had been fashioned. Huxley, who twenty years afterwards had occasion to subject the Old Red Sandstone fishes to critical study, and who brought to the inquiry all the resources of modern biology, has left on record the impression made on his mind by a minute revision of Hugh Miller's work. 'The more I study the fishes of the "Old Red,"' he remarks, 'the more am I struck with the patience and sagacity manifested in Hugh Miller's researches, and by the natural insight which in his case seems to have supplied the place of special anatomical knowledge.' He refers to the 'excellent restoration of _Osteolepis_,' in which even some of the minute peculiarities had not escaped notice, and he declares that Hugh Miller had made known almost the whole organisation of _Dipterus_, and had thus anticipated the most important part of Professor Pander's labours in the same field, the distinguished Russian palæontologist not having been aware that the work had already been done in Scotland.