Selachians

Sub-order I. Notidanoidea. Six or seven gill-openings. A single dorsal fin above the anal. Rostrum simple.

Family I. Chlamydoselachidae (Chlamydoselachus).

Family 2. Hexanchidae (Hexanchus, Heptanchus).

Sub-order 2. Galeoidea. Five gill-openings. Two dorsal fins and an anal ; no fin-spines. Rostrum triradiate.

Family I. Odontaspididae (Odontaspis, Scapanorhynchus). Family 2. Lamnidae (Lamna, Carcharodon, Alopias, Ceto rhinus).

Family 3. Orectolobidae (Orectolobus, Chiloscyllium, Stego stonea, Ginglymostoma, Rhinodon).

Family 4. Scyliorhinidae (Scyliorhinus, Pseudotriacis).

Family 5. Carchariidae (Carcharias, Sphyrna, Galeus, Mus telus).

Sub-order 3. Squaloidea. Five or six gill-openings. Two dorsal fins, each typically preceded by a spine, but if these are wanting the anal fin is also absent. Rostrum simple.

Family 1. Hybodontidae (Mesozoic).

Family 2. Heterodontidae (Heterodontus, Gyropleurodus). Family 3. Protospinacidae (Jurassic).

Family 4. Pristiophoridae (Pristiophorus, Pliotrema).

Family 5. Squalidae (Squalus, Spinax, Centrophorus, Som niosus).

Family 6. Squatinidae (Squatina).

Order 2. Hypotremi (rays) Pectoral fins produced forwards and joined to head; gill openings ventral. Hyomandibular purely suspensory.

Sub-order 1. Narcobatoidea. Large electric organs between pectoral fins and head. Suprascapulae united above vertebral column.

Family I. Torpedinidae (Torpedo, Narcine, etc.).

Sub-order 2. Batoidae. No large electric organs. Suprascapulae united to vertebral column.

Family I. Rhinobatidae (Rhinobatus, Discobatus). Family 2. Pristidae (Sclerorhynchus, Pristis).

Family 3. Raiidae (Raia, Psammobatis, etc.).

Family 4. Trygonidae (Trygon, Myliobatis, Rhinoptera, Mo bula, etc.).

Sub-class 5. Holocephali. Males with mixopterygia. Pterygo quadrate fused with skull. Lower jaw with one pair of tooth plates, upper with two pairs. Hyoid arch complete, with pharyngo hyal. Gill-clefts opening into a chamber with one external opening.

Family I. Squaloraiidae (Mesozoic).

Family 2. Myriacanthidae (Mesozoic).

Family 3. Chimaeridae (Chimaera, Callorhynchus, Harriotia, Rhinochimaera).

The three sub-classes distinguished by the presence of mix opterygia are evidently derived from a common ancestor, which was not hyostylic, for the presence of a complete hyoid arch in the Holocephali, with a pharyngo-hyal above the epi-hyal or hyomandibular, shows that they are not derived from a form in which the hyomandibular articulated with the skull and supported the jaws. In the Palaeozoic sub-classes the pterygo-quadrate ap pears to have articulated with the otic region of the skull; such an articulation occurs in the Hexanchidae and Hybodontidae of the Euselachii, but whether it is primitive or secondary in these is uncertain. Some authors consider the Holocephali to be related to the Squaloid sharks, but their tooth-plates appear to be formed by fusion, not enlargement, and are therefore not comparable with the enlarged lateral teeth of Heterodontus, and the dorsal fin spine is not an enlarged denticle as in the Squaloids, but is formed by concrescence of horny rays; moreover, the structure of the hyoid arch, and the separate pelves, preclude the derivation of the Holocephali from the Euselachii. For further details of the anatomical characters of the orders and families of the Euselachii Regan (1906) may be consulted.

Geographical Distribution.

Many families of Euselachii are restricted to tropical and subtropical seas, the principal exceptions being the Scyliorhinidae, Squalidae, Squatinidae and Raiidae, nearly all of which are bottom-living forms ; to these four families belong the species that can be reckoned as British, except summer migrants and occasional visitors. Squatina is found mainly north and south of the Tropics, as is Squalus, but Scyliorhinus and Raia are large, nearly cosmopolitan genera. A few sharks enter rivers, and two species of Carcharias are believed to live only in fresh water, one in Lake Nicaragua, the other in the Zambezi. Of the rays, Pristis and Trygon may ascend rivers, and there are a few fresh water Trygonidae in South America.