What, let us inquire, is the character of these ancient fishes, that first complete the scale of animated nature in its four master ideas, by adding the vertebrate to the invertebrate divisions? So far as is yet known, they all consist of one well marked order,--that placoidal order of Agassiz that to an internal framework of cartilage adds an external armature, consisting of plates, spines, and shagreen points of solid bone. Either of the two kinds of dog-fishes on our coasts,--the spiked or spotted,--maybe accepted as not inadequate representatives of this order as it now exists. The Port Jackson shark, however,--a creature that to the dorsal spines and shagreen-covered skin of the common dog-fish adds a mouth terminal at the snout, not placed beneath, as in most other sharks, and a palate covered with a dense pavement of crushing teeth,--better illustrates the order as it first appeared in creation than any of our British placoids. [Illustration: Fig. 52. PORT JACKSON SHARK. (Cestracion Phillippi.)] [Illustration: Fig. 53.[13] ----------------------- Silurian. -----+-----------------Placoid. | | Ganoid. Old Red. | | -----+----+------------ | | Carboniferous. | | -----+----+------------ Permian. | | -----+----+------------ Triassic. | | -----+----+------------ | | Oolitic. | | -----+----+----+----+-- Cretaceous. | | | | Ctenoid and Cycloid. | | | | -----+----+----+----+-- Tertiary. | | | | | | | | -----+----+----+----+-- Geologic [Pla. Gan. Cte. Cyc.] arrangement. Agassiz's [Pla. Gan. Cte. Cyc.] arrangement. THE GENEALOGY OF FISHES.] [Illustration: Fig. 54. AMBLYPTERUS MACROPTERTUS. From the Coal at Saarbruck. (A Ganoid of the Carboniferous System.)] [Illustration: Fig. 55. LEBIAS CEPHALOTES. Cycloids of Aix. (_Miocene._)] [Illustration: Fig. 56. PLATAX ALTISSIMUS. A Ctenoid of Monte Bolca. (_Eocene._)]