if, as is probable, this was attached to a powerful body, perhaps thirty feet in length, and capable of rapid motion through the water, we cannot imagine any creature so strong or so well armed as to cope with the mighty _Dinichthys_. [M] Many of these were discovered and successfully displayed and described by Hugh Miller, and are graphically portrayed in his celebrated work on the "Old Red Sandstone," published in 1841. The difference between the fishes of the Devonian and those of the modern seas is well marked by the fact that, while the ordinary bony fishes now amount to probably 9,000 species, and the ganoid fishes to less then thirty, the finny tribes of the Devonian are predominantly ganoids, and none of the ordinary type are known. To what is this related, with reference to conditions of existence? Two explanations, different yet mutually connected, may be suggested. One is that armour was especially useful in the Devonian as a means of defence from the larger predaceous species, and the gigantic crustaceans of the period. that this was the case may be inferred from the conditions of existence of some modern ganoids. The common bony pike of Canada (_Lepidosteus_), frequenting shallow and stagnant waters, seems to be especially exposed to injury from its enemies. Consequently, while it is rare to find an ordinary fish showing any traces of wounds, a large proportion of the specimens of the bony pike which I have examined have scars on their scales, indicating injuries which they have experienced, and which possibly, to fishes not so well armed, might have proved fatal. Again, in the modern Amia, or mud-fish, in the bony pike and _Polypterus_, there is an extremely large air-bladder, amply supplied with blood-vessels, and even divided into cells or chambers, and communicating with the mouth by an "air-duct." This organ is unquestionably in function a lung, and enables the animal to dispense in some degree with the use of its gills, which of course depend for their supply of vital air on the small quantity of oxygen dissolved in the water. Hence, by the power of partially breathing air, these fishes can live in stagnant and badly aerated waters, where other fishes would perish. In the case of the _Amia_, the grunting noises which it utters, its habit of frequenting the muddy creeks of swamps, and its possession of gill-cleaners, correspond with this view. It is possible that the Devonian fishes possessed this semi-reptilian respiration; and if so, they would be better adapted then other fishes to live in water contaminated with organic matter in a state of decay, or in waters rich in carbonic acid or deficient in oxygen. Possibly the palæozoic waters, as well as the palæozoic atmosphere, were less rich in pure oxygen then those of the present world; and it is certain that, in many of the beds in which the smaller Devonian fishes abound, there was so much decaying vegetable matter as to make it probable