Of the membranous-finned and thorny-rayed order of ichthyolites, the _Cheiracanthus_, or thorny-hand, (_i. e._ pectoral,) may be regarded as an adequate representative. (See Plate VII., fig. 1.) The _Cheiracanthus_ must have been an eminently handsome little fish--slim, tapering, and described in all its outlines, whether of the body or the fins, by gracefully waved lines. It is, however, a rare matter to find it presenting its original profile in the stone;--none of the other ichthyolites are so frequently distorted as the _Cheiracanthus_. It seems to have been more a cartilaginous and less an osseous fish than most of its contemporaries. However perfect the specimen, no part of the internal skeleton is ever found, not even when scales as minute as the point of a pin are preserved, and every spine stands up in its original place. And hence, perhaps, a greater degree of flexibility, and consequent distortion. The body was covered with small angular scales, brightly enamelled, and delicately fretted into parallel ridges, that run longitudinally along the upper half of the scale, and leave the posterior portion of it a smooth, glittering surface. (See Plate VII., fig. 2.) They diminish in size towards the head, which, from the faint stain left on the stone, seems to have been composed of cartilage exclusively, and either covered with skin, or with scales of extreme minuteness. The lower edge of the operculum bears a tagged fringe, like that of a curtain. The tail, a fin of considerable power, had the unequal sided character common to the formation; and the slender and numerous rays on both sides are separated by so many articulations as to present the appearance of parallelogramical scales. The other fins are comparatively of small size. There is a single dorsal placed about two thirds the entire length of the creature adown the back; and exactly opposite its posterior edge is the anterior edge of the anal fin. The ventral fins are placed high upon the belly, somewhat like those of the perch; the pectorals only a little higher. But it is rather in the construction of the fins, than their position, that the peculiarities of the _Cheiracanthus_ are most marked. The anterior edge of each, as in the pectorals of the existing genera _Cestracion_ and _Chimæra_, is formed of a strong, large spine. In the _Chimæra borealis_, a cartilaginous fish of the Northern Ocean, the spine seems placed in front of the weaker rays, just, if I may be allowed the comparison, as, in a line of mountaineers engaged in crossing a swollen torrent, the strongest man in the party is placed on the upper side of the line, to break off the force of the current from the rest. In the _Cheiracanthus_, however, each fin seems to consist of but a single spine, with an angular membrane fixed to it by one of its sides, and attached to the creature's body on the other. Its fins are masts and sails--the spine representing the mast,