MURÆNIDÆ-- Nemichthys scolopacea „ infans 500–2500 Cyema atrum 1500–1800 Saccopharynx Synaphobranchus pinatus 345–1200 „ bathybius 1875–2050 „ brevidorsalis 1075–1375 „ affinis 345 Nettastoma parviceps 345 CYCLOSTOMATA-- Myxine australis 345 [Illustration: Fig. 111.--_Chiasmodus niger_; obtained in the North Atlantic at a depth of 1500 fathoms; the specimen has swallowed a large Scopelus (_s_); _o_, ventral fin.] 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_. FIRST ORDER: CHONDROPTERYGII.