covered greatly diminished prolongations of the spinal cord. In the base of the Secondary division,—another long stage towards the existing state of things,—we find, with the homocercal tail, which now appears for the first time, numerous tails like that of the _Lepidosteus_, (fig. 51,) of an intermediate type;—they are rather tails set on awry than truly heterocercal. The diminished cord has disappeared from among the fin rays. In the numerous Lepidoid genus, and the genera _Semionotus_ and _Tetra gonolepis_,—all ganoidal fishes of the Secondary period—this intermediate style is very marked; while in their contemporaries of the genera _Uræus_, _Microdon_, and _Pycnodus_, we find the earliest examples of true homocercal tails. And in the Ctenoids and Cycloids of the Chalk the homocercal tail receives its fullest development. It finds bases for its rays in broad non-homological processes, that spread out behind abruptly-terminating vertebræ, (fig. 52,) in the same period in which, by a strange process of degradation, the four ichthyic limbs are first gathered into a cluster, and hung about the neck.[30] [Illustration: Fig. 52. TAIL OF PERCH.] I am aware that by some very distinguished comparative anatomists, among the rest Professor Owen, the attachment, so common among fishes, of the scapular arch and the fore limbs to the occipital bone, is regarded, not as a displacement, but as a normal and primary condition of the parts. Recognizing in the scapular bones the _ribs_ of the occipital _centrum_, the anatomists of this school of course consider them, when found articulated to the occiput, as in their proper and original place, and as in a state of natural dislocation when removed, as in all the reptiles, birds, and mammals, farther down. We find Professor Oken borrowing support to his hypothesis from this view. The limbs, he tells us, are simply ribs, that in the course of ages have been set free, and have become by development what they now are. And it is unquestionably a curious and interesting fact, that there are certain animals, such as the crocodile, in which every centrum of the vertebral column, and of every _vertebra_ of the head, has its ribs or rib-like appendages, with the exception of the occipital _centrum_. And it is another equally curious fact, that there is another certain class of animals, such as the osseous horn-covered fishes, with the Sturionidæ, Salamandroidei, and at least one genus among the Placoids, (the Chimæroidei,) in which this occipital centrum bears as its _ribs_ the scapular bones, with their appendages the fore limbs. It is the _centrum_ without _ribs_ that is selected in these animals as the centrum to which the scapular _ribs_ should be attached. Be it remembered, however, that while it is unquestionably the part of the comparative anatomist to determine the relations and homologies of those parts of which all animals are composed, and to interpret the significancy in the scale of being of the