Tetradactylous birds, or those which have four toes, have only three of them directed forwards, and the fourth backwards, generally. There are, however, exceptions; some birds have four toes directed forwards: this is the fact with the Hirundo Cypselus and the Pelicanus Aquilus of Linnaeus, or Man-of-war Bird. But the articulations are different in the two animals, birds having regularly two, three, four, and five phalanges, and the spur, where it exists, supported by a single osseous phalanx; whereas the Otozoum has three phalanges in the inner and second toe, four in the third and fourth toes. In this last arrangement, the Otozoum is decidedly different from all known birds. It is not likely to have been a tortoise or a lizard. The kangaroo has four feet, and uses only two in progression, moving forward by leaps; also, like the Otozoum, it has four toes; but the size of the toes does not accord with that of the Otozoum, nor is the structure of the foot the same, so far as we know. It has been suggested by Professor Agassiz, that this animal might have been a two-footed frog. Nature had, in those days, animal forms different from those we are acquainted with; and this might have been the fact with the Otozoum. * * * * * GROUP SIXTH. We have in this group a specimen of the track of a four-footed animal, which may have been a frog, though different from ours. The feet are unequal in size, and present a different number of toes. In existing frogs there are four toes in the fore feet, and five in the hind; but, in the specimen before us, the front toes are five in number, and the back toes three. It is called, therefore, ANOMOEPUS, _unequal-footed_. These impressions are in the red shale of Hadley, and very distinct. In some of them the lower leg is indicated, forming an impression six or seven inches long. The feet being smaller than the legs, the impression made by the latter is more expanded, superficial, and broader, yet still very definite. The opinion of President Hitchcock and Dr. Deane is, that the different impressions of five and three toes are those of the anterior and posterior extremities of one animal, which, from the size of the limbs, might be a frog three feet high. On the same schist with these footmarks, are other curious impressions. The back of the slab is almost covered with the imprints of rain-drops. In the midst of these is a tridactylous impression, probably of a quadruped, crossed at its root by a single depression, nearly an inch broad, and two and a half long: this seems to form part of another broad superficial impression of about seven by four inches, which is probably also quadrupedal. Other parts present the impressions of nails and worm-tracks. At the opposite end is a deep, smooth, regular excavation, which might have been made by a Medusa. * * * * * GROUP SEVENTH.