I was struck, during my explorations at this time, as I had been often before, by the style of grouping, if I may so speak, which obtains among the Lower Old Red fossils. In no deposit with which I am acquainted, however rich in remains, have all its ichthyolites been found lying together. The collector finds some one or two species very numerous; some two or three considerably less so, but not unfrequent; some one or two more, perhaps, exceedingly rare; and a few, though abundant in other localities, that never occur at all. In the Cromarty beds, for instance, I never found a Holoptychius, and a Dipterus only once; the Diplopterus is rare; the Glyptolepis not common; the Cheirolepis and Pterichthys more so, but not very abundant; the Cheiracanthus and Diplacanthus, on the other hand, are numerous; and the Osteolepis and Coccosteus more numerous still. But in other deposits of the same formation, though a similar style of grouping obtains, the proportions are reversed with regard to species and genera: the fish rare in one locality abound in another. In Banniskirk, for instance, the Dipterus is exceedingly common, while the Osteolepis and Coccosteus are rare, and the Cheiracanthus and Cheirolepis seem altogether awanting. Again, in the Morayshire deposits, the Glyptolepis is abundant, and noble specimens of the Lower Old Red Holoptychius--of which more anon--are to be found in the neighborhood of Thurso, associated with remains of the Diplopterus, Coccosteus, Dipterus, and Osteolepis. The fact may be deemed of some little interest by the geologist, and may serve to inculcate caution, by showing that it is not always safe to determine regarding the place or age of subordinate formations from the per centage of certain fossils which they may be found to contain, or from the fact that they should want some certain organisms of the system to which they belong, and possess others. These differences may and do exist in contemporary deposits; and I had a striking example, on this occasion, of their dependence on a simple law of instinct, which is as active in producing the same kind of phenomena now as it seems to have been in the earlier days of the Old Red Sandstone. The Cromarty and Moray Friths, mottled with fishing boats (for the bustle of the herring fishers had just begun), stretched out before me. A few hundred yards from the shore there was a yawl lying at anchor, with an old fisherman and a few boys angling from the stern for sillocks (the young of the coal-fish) and for small rock-cod. A few miles higher up, where the Cromarty Frith expands into a wide landlocked basin, with shallow sandy shores, there was a second yawl engaged in fishing for flounders and small skate,--for such are the kinds of fish that frequent the flat shallows of the basin. A turbot-net lay drying in the sun: it served to remind me that some six or eight miles away, in an opposite direction, there is a deep-sea bank, on which turbot, halibut, and large skate are found. Numerous boats were stretching down the Moray Frith, bound for the banks of a more distant locality, frequented at this early stage of the herring fishing by shoals of herrings, with their attendant dog-fish and cod; and I knew that in yet another deep-sea range there lie haddock and whiting banks. Almost every variety of existing fish in the two friths has its own peculiar habitat; and were they to be destroyed by some sudden catastrophe, and preserved by some geologic process, on the banks and shoals which they frequent, there would occur exactly the same phenomena of grouping in the fossiliferous contemporaneous deposits which they would thus constitute, as we find exhibited by the deposits of the Lower Old Red Sandstone.