A t the request of my friend Professor Ramsay I made an examination of the fossils collected last summer by Mr. Geikie from the Lias of Pabba, Scalpa, and Strath; first, for the purpose of determining the species to which they belonged; and, secondly, to assign the subdivisions of the formation which they characterized. Previous to laying before the Geological Society my notes on the different species contained in this interesting collection, it is necessary that I should premise a few remarks on the classification of the Lias, in order that I may rigorously define the portion of the formation I include in the term middle ; so that the true stratigraphical position of the beds which yielded these organisms may be fixed with certainty. English authors in general divide the Lias into Upper Lias, Marlstone , and Lower Lias ; but these subdivisions require additions and modifications in order to place our liassic beds in correlation with those of French and German authors. For on the Upper Lias clays are superimposed beds, which, previously to the publication of my memoir on the “Upper Lias Sands”*, were grouped with the Inferior Oolite; and with the Lower Lias are placed several beds which, with the Marlstone, constitute the Middle Lias of the Continental authors. Taking the Lias-beds as they occur in Gloucestershire, in descending order, and naming each bed by the species of Ammonite which characterizes it, we find the series tabulated at p. 25, which is nearly the equivalent of the Lias-beds of