It is well known to those who have paid attention to the progress of Fossil Ichthyology that, until the publication of M. Agassiz, the distinctive characters of the orders, genera, and species of Fossil Fishes were but imperfectly understood. Vague analogies were relied on to connect them with the types of living genera, and the looseness of the received specific characters rendered it difficult for the geologist to determine whether the specimens he collected were previously recognised, or still nondescript. It is obvious that useful characters of fossil species are chiefly to be obtained from those portions of their structure least subject to alteration from decay; and as the exterior scaly envelopes of the primeval fishes are usually the portions best preserved and most easily recognised in their rocky sepulchres, M. Agassiz was naturally led to study these with minute attention. This acute observer speedily discovered that, in the form and connections of the scales, he had a general character which would enable him to connect into very natural groups, species differing from each other in size and form. On this basis he has established his four Orders of Fossil Fishes—the Ganoidei, the Placoidei, the Ctenoidei, and the Cyclodei—divisions named from the appearance of the scales.