_Skeleton cartilaginous. Body with medial and paired fins, the hinder pair abdominal. Vertebral column generally heterocercal, the upper lobe of the caudal fin produced. Gills attached to the skin by the outer margin, with several intervening gill-openings: rarely one external gill-opening only. No gill-cover. No air-bladder. Two, three, or more series of valves in the conus arteriosus. Ova large and few in number,[33] impregnated and, in some species, developed within a uterine cavity. Embryo with deciduous external gills.[34] Males with intromittent organs attached to the ventral fins.[35]_ This order, for which, also, the name _Elasmobranchii_ has been proposed (by Bonaparte), comprises the Sharks and Rays and Chimæras, and is divided into two suborders: _Plagiostomata_ and _Holocephala_. FIRST SUB-ORDER: PLAGIOSTOMATA. _From five to seven gill-openings. Skull with a suspensorium and the palatal apparatus detached. Teeth numerous._ The Plagiostomes differ greatly among each other with regard to the general form of their body: in the Sharks or _Selachoidei_ the body is elongate, more or less cylindrical, gradually passing into the tail; their gill-openings are lateral. In the Rays, or _Batoidei_, the gill-openings are always placed on the abdominal aspect of the fish; the body is depressed, and the trunk, which is surrounded by the immensely developed pectoral fins, forms a broad flat disk, of which the tail appears as a thin and slender appendage. Spiracles are always present; the number of gill-openings is constantly five; no anal fin; dorsal fins, if present, situated on the tail. However, some of the Rays approach the Sharks in having the caudal portion less abruptly contracted behind the trunk. Fossil Plagiostomes are very numerous in all formations. Some of the earliest determinable fish remains are believed to be, or are, derived from Plagiostomes. Those which can be referred to any of the following families will be mentioned subsequently: but there are others, especially fin-spines, which leave us in doubt to which group of Plagiostomes their owners had any affinity, thus _Onchus_ from the upper Silurian, continuing to carboniferous formations; _Dimeracanthus_, _Homocanthus_, from the Devonian; _Oracanthus_, _Gyracanthus_, _Tristychius_, _Astroptychius_, _Ptychacanthus_, _Sphenacanthus_, etc., from carboniferous formations; _Leptacanthus_, from the coal to the Oolite; _Cladacanthus_, _Cricacanthus_, _Gyropristis_, and _Lepracanthus_, from the coal measures; _Nemacanthus_, _Liacanthus_, from the Trias; _Astracanthus_, _Myriacanthus_, _Pristacanthus_, from the Jurassic group. A. SELACHOIDEI: SHARKS.