the head of pterichthys, as movable as fins, have easily given rise to the variety of opinions concerning the true affinity of these singular creatures, and has caused them to be taken at one time for gigantic coleoptera, at another for crustacea, or small marine tortoises; so little do the types of the classes appear fixed in certain respects at these remote times. Another singularity of these genera is the association to the bony plates of the head of a vertebral appendage, which is far from having acquired the same solidity; but appears, on the contrary, to have remained fibro-cartilaginous during the whole life of the animal—resembling in this respect the skeleton of the sturgeon. “It would be difficult to find among recent fishes, types presenting any direct analogy with the genera pterichthys, coccosteus, and cephalaspis; it is only from afar that they can be compared to some abnormal genera of our epoch.... The analogy which they offer, on the one hand, in form with the dorsal cord of the embryo of fishes, together with the inferior position of their mouth, which is equally met with in the embryos; and on the other hand, the distant resemblance of these fishes to certain types of reptiles, present the most curious assemblage of characters that can possibly be conceived. A third group of fishes belonging to this formation, comprises those genera whose vertical fins are double on the back and under the tail, and which approach very near to the caudal. These are the genera dipterus, osteolepis, diplopterus, and glyptolepis, which differ from one another by the form of their scales and their dentition. And lastly, it seems necessary to regard as a fourth group of this order, the genera which are characterized by large conical teeth, situated on the margin of the jaws, between which are alternately smaller, and indeed very small ones, in the form of a brush. Such are the genera holoptychius and platygnathus, and the genus recently established by Mr. Owen under the name of dendrodus, and respecting which this learned anatomist has given some exceedingly interesting microscopical details.”