This paper is based upon a restudy of Traquair''s original material and upon new collections recently made at the type locality. A review of the Scottish Thelodidae is included and a new occurrence of Thelodus (Ordovician) from Canyon City, Colorado, is noted. The genera Thelodus and Lanarkia from the Silurian of Scotland are reviewed and descriptions of structure of the mouth, position of orbit, and structure of exo-skeleton given of the following spp. T. scotius*; T. planus*; T. taiti* (p. 143), Downtonian, type in Scottish Surv. coll.; Lanarkia horrida* (tail also described) ; Drepanaspis gemundenensis*. Possible elasmobranch affinities for the Heterostraci are suggested. But the mouth structure, position of orbits, and structure of exoskeleton afford evidence precluding any close relationship with the Caphalaspidae.