tricht beds, to which we have referred above, as well as in the method of replacement, for some of the teeth preserved have the crowns of their successors implanted in cavities in their fangs. This peculiarity in the manner of teeth-shedding is characteristic of some reptiles, each of whose teeth is merely a hollow cone tilled in the interior with a soft pulp which supplies the material for the external bony layer. When the tooth becomes worn and useless, a new one is formed beneath the shell of the first by the pulp in the interior, which gradually ousts the old from its socket. In addition to these, the hesperornis possessed other reptilian characters. While the formation of the spinal column in the neck and back is of the true avian type, the structure of the tail, where there have been discovered no fewer than twelve segments, is very peculiar, and differs entirely from anything hitherto seen in birds. The bones of its middle and posterior portions have very long and horizontally flattened processes which prevent all motion in a lateral direction: a peculiarity from which we may certainly infer that, like the beaver's, this appendage was moved vertically, and doubtless was an efficient aid in diving, perhaps compensating for want of wings, which the penguins use with such wonderful dexterity in swimming under water. The last three or four bones are firmly united together, forming a flat terminal mass analogous to, but quite unlike, the "ploughshare" bone of modern birds.
Here, again, is another form half doubtful whether to assume the reptilian or the avian garb, a protestant against the hard and fast lines within which the various groups of the animal kingdom have hitherto been confined. The hesperornis certainly approaches the ichthyornis so far as to come under the new sub-class instituted for the reception of that bird; but, inasmuch as it differs in having its teeth not in sockets but set in a groove, and since, rejecting the conservative bi-concavity in the matter of spinal segments, it has adopted a newer and more high-class "cut," it has been necessary to give to each the honor of heading a separate section.
Though no living bird has so long a tail as this bird-of-the-dawn, yet there was in 1862 disinterred from the lithographic slates of Solenhofen part of the skeleton of a feathered biped—the archæopteryx (the existence of which was foreshadowed by the discovery of a feather the year before), exhibiting in most of the bones preserved the marks of a true bird. In the length of its tail, however, it is peculiar. This appendage contains the enormous number of twenty distinct bones gradually decreasing in size to the last, and each supporting a pair of quill feathers. To the skeleton no head is attached; but a portion of a small separate jaw on the same slab has been the subject of much controversy as to whether it belongs to the accompanying bones or not. Hermann von Meyer, the illustrious anatomist and paleontologist, holds that there can be little doubt but that they are parts of one and the same skeleton. If this be so, these remains belonged to a toothed bird; and