The yery fine specimen of palatal tooth of Ptychodus polygyrus, which we figure in Plate IX., from the collection of N. T. Wetherell, Esq., of Highgate, temporarily draws our attention to a class of remains of very considerable interest.

We have not the leisure at the present time for going as deeply into the subject as it well deserves, nor as the mass of valuable materials accumulated since the publications of Agassiz in 1843, and Dixon in 1850, require.

There are also other important points than the mere bearings of more detailed information of the characters of species very possibly to be gained by a study of the singular and marked group of cestraciont fishes. First known, in abundance of individuals, in the Carboniferous age—though not at any time numerous in genera,—and presenting various forms, numerically abundant, in the Jurassic and other intermediate formations up to the Chalk, characterized by its many varieties of Ptychodus, but now dwindled down to a solitary representative in the Port Jackson shark, it is one of those very circumscribed groups in which we ought to find more especially and distinctly marked traces of the transmutation of one species into another, if such transmutation did exist in the past ages of our planet. That the group does present important evidence on this point is certain, but whether sufficient or not to come to a practical and definite conclusion, may be as yet doubtful; although, if collectors will turn to the fossil remains of these fishes in earnest, we may rest assured of good work in this direction being done. By a glance at the British Museum specimens, and a careful looking over of the descriptions and figures in the ‘Poissons Fossiles’ and the ‘Geology of Sussex,’ any intelligent observer would at once see what new additions would be useful for supplying the missing links in the historic and stratigraphical series. We add here a list of the species of Ptychodus exhibited in our National Collection.