CLASS 1IL-AVES.
Long before any evidence of birds from actual or recog nizable fossil remains is obtained in tracing the progress of life from the oldest fossiliferous deposits upwards, we meet with indications of their existence impressed in sandstones of the triassic or liassic period.
These earliest evidences of the class are by footprints in some former tidal shore, preserved in one or other of the ways explained in the section " Ichnology." The fossil bones of birds have not been found save in strata of much later date than the impressed sandstones ; and they are much more rare than the remains of mammals, reptiles, and fishes, in any formations except the most recent in certain limited localities, Sir C. Lyell has well remarked, that " the powers of flight possessed by most birds would insure them against perishing by numerous casualties to which quadrupeds are exposed during floods." The same writer further argues, that "if they chance to be drowned, or to die when swimming on water, it will scarcely ever happen that they will be submerged so as to become preserved in sedimentary deposits."* It is true that the carcase of a floating bird may not sink where it has died, but be carried far along the stream : ultimately, however, if not devoured, its bones will subside when the soft parts have rotted, and both the compactness of the osseous tissue, and the facts made known by the ornitholites of the greensand near Cambridge, of the London clay at Sheppy, and of the Montmartre eocene quarry-stone, show that they can be pre served in the fossil state. The length of time during which the carcase of a bird may float, doubtless exposes it the more to be devoured, and so tends to make more scarce the fossil remains of birds in sedimentary strata. Certain it is that the major part of the remains of extinct birds that have as yet been found are those of birds that were deprived of the power of flight, and were organized to live on land.
The existence of birds at the triassic period in geology, or at the time of the formation of sandstones which are certainly intermediate between the has and the coal, is indicated by abundant evidences of footprints impressed upon those sand stones which extend through a great part of the valley of the Connecticut River, in Connecticut and Massachusetts, North America.
The footprints of birds are peculiar, and more readily dis tinguishable than those of most other animals. Birds tread
on the toes only ; these are articulated to a single metatarsal bone at right angles equally to it, and they diverge more from each other, and are less connected with each other, than in other animals, except as regards the web-footed order of birds.
Not more than three toes are directed forward ;* the fourth when it exists, is directed backward, is shorter, usually rises higher from the metatarsal, and takes less share in sustaining the superincumbent weight. No two toes of the same foot in any bird have the same number of joints. There is a constant numerical progression in the number of phalanges (toe-joints), from the innermost to the outermost toe. When the back toe exists, it is the innermost of the four toes, and it has two phalanges, the next has three, the third or middle of the front toes has four, and the outermost has five phalanges. When the back toe is wanting, as in some waders, and most wingless birds, the toes have three, four, and five phalanges respectively. When the number of toes is reduced to two, as in the ostrich, their phalanges are respectively four and five in number ; thus showing those toes to answer to the two outermost toes in tridactyle and tetradactyle birds.
The same numerical progression characterizes the two phalanges in most lizards from the innermost to the fourth ; but a fifth toe exists in them which has one phalanx less than the fourth toe. It is the fifth toe which is wanting in every bird. In some Gallinacea, one or two (Pavo bicalca ratus) spurs are superadded to the metatarsus ; but this peculiar weapon is not the stunted homologue of a toe. Dr. Deane of Green field, United States, noticed, in 1835, impressions resembling the feet of birds in some slabs of sandstone from Connecticut River, and first, in a letter to Dr. Hitchcock, dated March 7, 1835, recorded his belief that they were the footsteps of a bird. He prepared casts of the impressions, some of which he trans mitted, with his opinion of their nature, to Professor Silliman, Editor of the " American Journal of Science," in April 1835. Dr. Hitchcock, President of Amherst College, United States, first made public the fact, based on scientific comparison, and sub mitted to the geological world his interpretations of those impressions as having been produced by the feet of living birds, and he gave them the name of Ornithichnite,s.