SYSTEMATIC AND DESCRIPTIVE PART.
The Class of Fishes is divided into four sub-classes:—
I. Palæichthyes.—Heart with a contractile conus arteriosus; intestine with a spiral valve; optic nerves non-decussating, or only partially decussating.
II. Teleostei.—Heart with a non-contractile bulbus arteriosus; intestine without spiral valve; optic nerves decussating. Skeleton ossified, with completely separated vertebræ.
III. Cyclostomata.—Heart without bulbus arteriosus; intestine simple. Skeleton cartilaginous and notochordal. One nasal aperture only. No jaws; mouth surrounded by a circular lip.
IV. Leptocardii.—Heart replaced by pulsating sinuses; intestine simple. Skeleton membrano-cartilaginous and notochordal. No skull; no brain.
FIRST SUB-CLASS: PALÆICHTHYES.
Heart with a contractile conus arteriosus;[30] intestine with a spiral valve;[31] optic nerves non-decussating, or only partially decussating;[32] skeleton cartilaginous or osseous.
This sub-class comprises the Sharks and Rays, and the Ganoid fishes. Although based upon a singular concurrence of most important characters, its members exhibit as great a diversity of form, and as manifold modifications in the remainder of their organisation as the Teleostei. The Palæichthyes stand to the Teleostei in the same relation as the Marsupials to the Placentalia. Geologically, as a sub-class, they were the predecessors of Teleosteous fishes; and it is a remarkable fact that all those modifications which show an approach of the ichthyic type to the Batrachians are found in this sub-class. We divide it into two orders: Chondropterygii and Ganoidei.