mucoidal in _Spinax_, _Scyllium_, and _Cestracion_—have become osseous in the sturgeon. The _fish_ has _set_ and become _fixed_, as cement sets in a building, or colors are fixed by a mordant. And it is worthy of special remark that, correspondent with the peculiarly _ichthyic_ development of tail in this fish, we find the prevailing ichthyic displacement of the fore limbs. Again, in the _Lepidosteus_, another of the true Ganoids which still exist, the internal angle of the upper lobe of the tail wholly disappears, and with the internal angle the prolongation of the nervous cord. Still, however, it is what the tail of the sturgeon would become were the angular patch to be obliterated, and rays substituted instead,—it is a tail set on awry. And in this fish also we find the ichthyic displacement of fore limb. One step more, and we arrive at the homocercal or equal-lobed tail, which seems to attain to its most extreme type in those fishes in which, as in the perch and flounder, the last vertebral joint, either very little or very abruptly diminished in size, expands into broad processes without homologue in the higher animals, on which the caudal rays rest as their bases. And in by much the larger proportion of these fishes all the four limbs are slung round the neck;—they at once exhibit the homocercal tail in its broadest type, and displacement of limb in its most extreme form. [Illustration: Fig. 50. TAIL OF OSTEOLEPIS.] [Illustration: Fig. 51. TAIL OF LEPIDOSTEUS OSSEUS.]