The fourth species, still smaller, is the Brontozoum Gracillimum. On this slab the impressions are in relief; viz.: 1st, of Brontozoum Gracillimum; 2d, of Brontozoum Parallelum; 3d, of the track of a tortoise, fourteen inches long, and two wide. Other extensive eminences and depressions, with rain-drops, may be observed on the same surface. The fifth species is called BRONTOZOUM PARALLELUM, from the tracks being on a line with each other. Of this there are two specimens, one of them, however, being a single track. On the surface of the other slab there are at least five distinct tracks, one of them being a small new and undescribed species,--thus making the whole number of species of Brontozoum which we possess to be at least six. The SECOND GENUS of Struthiones is called _AEthyopus_, from _aithuia_, a _gull_, and _pous_, a _foot_,--gull-footed. This genus is smaller than the Brontozoum Giganteum; and we have two species, viz. the AETHYOPUS LYELLIANUS, which is the larger, and two specimens of AETHYOPUS MINOR. All of these are distinguished from the preceding genus by the winged foot, and in the Lyellianus by the shallowness of the impression. The AEthyopus Minor is not always distinguished by the superficiality of its impression. This is sometimes deep. Therefore this character may not be considered a distinctive one, or the AEthyopus Minor might be referred to another genus. Of the two specimens of this latter species, the first is in depression, tridactylous. The depressions are deep with rain-drops, marks of quadrupeds and zoophytes over the whole surface. The ornithichnic impressions are two in number; one superficial, the other very deep. The reversed surface of this slab contains one tridactylous impression in relief. The second specimen has three depressions; two of which are superficial, and the third is quite deep, displaying, by a depressed surface, the webbed character of the foot. * * * * * GROUP SECOND. We shall take, to characterize this group, the _Argozoum_, from _arges_, _swift_, _winged_. Of this genus there are two species, the larger of which is the ARGOZOUM DISPARIDIGITATUM. It is leptodactylous, and remarkable for the length of the middle toe. We have another species, which is smaller than the last named, and in which the toes are nearly of equal length; hence called ARGOZOUM PARIDIGITATUM. The other genus of this group is the PLATYPTERNA, and our specimen is named _Deaniana_. This genus is remarkable for the width of the heel; hence the name, from _platys_, _broad_, and _pterna_, _a heel_. It has three toes like the other genera of this group. * * * * * GROUP THIRD. This and the succeeding group are tetradactylous; having one toe behind, three forwards.