In this paper a detailed description is given of a species of Pygopterus (P. Greenockii) from the carboniferous shales of Wardie, Mid-Lothian, which was named by Agassiz, but without any figure or description, beyond the mention of the fact that the scales of the anterior part of the trunk are higher than broad, a circumstance distinguishing it from all the other species of this genus.

According to specimens exhibited by the author, the scales are of different forms on different parts of the body, being very minute, and nearly equilateral along the belly; the fins are large, and the dorsal is placed so far back as to be nearly opposite the anal; the interspinous bones of the azygos fins are well developed, and there are traces of vertebral apophyses, but none of vertebral bodies.