4 NATURAL HISTORY. 



membrane. Indeed, this bony structure cannot truly be called a brain-case, since it is merely a, 

 covering on the outside of the cartilage which contains the brain. The arches appended to this 

 cranium are a single strong pedicle on each side, which is articulated to an angular posterior process, 

 and has attached to it the arch which is called the mandible, or lower jaw, and the arch which 

 is considered to correspond to the tongue-bone in reptiles and birds, and is hence called the hyoid. 

 The maxillary arch, or upper jaw, is closely joined to its fellow in front by a ligament, and is 

 attached by ligaments to the anterior part of the cranial region, and is prolonged backward so as 

 to articulate with the lower jaw. Each lateral half of the jaw consists of a single cartilage, or 

 cartilage sheathed in bone, and thus far is comparable only with the jaws in mammals. According 

 to Sir R. Owen, there are also cartilages which represent the palatine and pterygoid bones in 

 the Monk-fish and in a Brazilian Torpedo. Four or five cartilaginous rays diverge from the hinder 

 margin of the pedicle which supports the jaws, and have stretched upon them a membrane which 

 corresponds to the operculum in bony fishes. The hyoid arch in the Shark family usually consists of 

 two long and strong lateral pieces, which are united to a middle symmetrical piece below, which is 

 flattened, termed the basi-hyoid. The two lateral portions, which rise from this like horns, and 

 are hence termed the cerato-hyoids, give off short cartilaginous processes from their hinder margins, 

 which correspond to the bones which are termed branchiostegal rays in bony fishes, and support 

 the outer membrane of the sac which contains the gills. The five branchial arches which extend 

 backward behind the hyoid arch are suspended from the sides of the front vertebrae of the trunk, 

 just as in the Lamprey. In Sharks three strong cartilages are prolonged forward from the lieaclj 

 which coalesce in the middle line and form the remarkable snout. In these fishes the shoulder 

 girdle is suspended a little behind the head. The ear is contained in a cartilaginous capsule 

 in the walls of the cranium, the eye is united by a cartilaginous pedicle with the orbit, 

 and the nasal sacs are arched over by nasal processes from the skull. Another modification of skull 

 is seen in the Lepidosiren, where the separate bones of the skull are distinctly formed. The fibrous 

 sheath of the notochord is ossified at the anterior end, and the ex-occipital bones rise from it, and 

 expand and converge so as to meet above the foramen magnum. A large cartilaginous capsule 

 surrounds the internal ear, but there is a long basi-cranial bone with cartilaginous plates at the 

 sides. Other bones of the upper part of the skull also have representatives. Each branch of the 

 lower jaw consists now of two pieces the hinder portion, termed the articulai', and the anterior, 

 termed the dentary, though the two dentary pieces have become united together in the middle line in 

 front. There are some slight representatives of the opercular arch. There is a single cerato-hyoid bone 

 on each side, but no basi-hyoid. In one of his lectures Sir R. Owen remarked of this fish : " I 

 believe it to manifest, upon the whole, the highest grade which is attained in the class of fishes, or 

 in the direct progress to perfection in what may be termed the vertebrate high road. The true or 

 typical osseous fishes deviate from this road into bypaths of their own, and superadd endless com- 

 plexities, of which we shall seek in vain for homologous parts in reptiles, birds, or mammals. The 

 Lepidosiren's skeleton presents the closest resemblance to that of the lowest class of reptiles, though 

 it differs therefrom both by a little less and a little more development." The skull in osseous fishes 

 is altogether different. It may be convenient, as the modifications which it presents are so multi- 

 tudinous, to bear in mind as one type the skull of the Codfish (Gadus morrhuci), which is one of the 

 largest and commonest British species. The head is larger in proportion to the trunk in fishes than 

 in any other class of animals. It is more or less conical. The base of the cone joins the trunk 

 without any intervening neck ; the jaws are usuall} 7 at the apex of the cone, which is flattened above, 

 and has the sides more or less converging below. The eyes are large, and the orbits communicate 

 with each other. There are two lateral fissures behind the head, which are called the gill-apertiires, 

 and are opened and closed by special mechanism. Besides receiving the food, the mouth takes in 

 streams of water for respiration, which, after bathing the gills, escape by the gill-apertures or 

 openings behind the operculum. The head also contains the heart and the whole of the breathing 

 organs, and the anterior limbs are often in very close union with the skull. There are more bones 

 in the head of a fish than in the head of any other animal. Most of these bones unite with 

 each other by ovei'lapping, like scales. The brain is contained in a cranial cavity, so that the bones 

 fit closely upon it. The upper sm-face of the head is often mai-ked by longitudinal crests, but 



