On the other hand, the scale of organization descends to a lower point in the Cartilaginous than any reached in the Osseous Orders. In the extreme forms the skeleton becomes obsolete; the spine is no longer divided into vertebræ, but is reduced to a soft, flexible, transparent, and slender column or thread of cartilage; and the animals can only with the utmost difficulty be distinguished from Worms.
Thus it seems probable that in a truly natural arrangement, as the illustrious Cuvier has suggested, the Cartilaginei ought not to be placed either above or below other Fishes, but rather as forming a parallel series, or Sub-Class, as the Marsupialia form a series of Mammalia, parallel with the Placentalia.
The Cartilaginous Fishes are not very numerous, when compared with the other Orders: they are, however, widely scattered, some of them being found in all seas, from the equator to the seas surrounding either pole. They are almost exclusively marine; the Sturgeons and the Lampreys, however, are exceptions, inhabiting rivers. Five Families are included in this Order, named, respectively, Acipenseridæ, Chimæradæ, Squalidæ, Raiadæ, and Petromyzonidæ.
Family I. Acipenseridæ.
(Sturgeons.)
The Sturgeons have their gills free, like other fishes, with a single opening, which is comparatively wide, and protected by a large, oval, radiated plate, as a gill-cover; there are no gill-
