212 



Introduc- 

 tion. 

 Osteology. 



ICHTHYOLOGY. 



high plates, while in low and flat heads their longitudinal 

 axis is the longest. In skulls of the ordinary form, they are 

 sub-circular, and are perforated as in the Carp, or this per- 

 foration may be replaced by a dee)) anterior notch, as in the 

 Cod. Its connections vary with its relative size, and, 

 according as the petrosal, which interposes between it and 

 the ex-occipital, is greater or smaller or wholly wanting. 

 The parietuls or spine of this arch are most commonly two 

 in number. In the Cyprinoid and Salamandroid fishes, 

 they unite above by a longitudinal sutiu'e (the sagittal); in 

 the Cyprinoid Catastomi a mesial rectangular cartilaginous 

 plate interposes between the ossified side plates ; in the 

 Salnionoids they soon coalesce ; and in some Siluroids they 

 coalesce with the supra-occipital as well as with each other. 

 In the Pike, Perch, and Cod, and most osseous fishes, the 

 parietals are separated from each other bv an anterior pro- 

 longation of the supra-occipital. The mastoids or parajjo- 

 physes of the ])arietal vertebra are interposed between the 

 ali-sphenoids and parietals, and project outwards and back- 

 wards farther than the par-occipitals, forming the second 

 strong transverse process at the side of the cranium. This 

 process is developed from the outer margin of the bone. 

 The inner side of the bone lodges part of one of the semi- 

 circular canals. The great cavity named "otocrane," 

 which lodges the proper acoustic capsule or petrosal, whether 

 osseous or cartilaginous, is formed by the ex-occipital, par- 

 occipital, ali-sphenoid, mastoid, and sometimes the parietal 

 and supra-occipital. 



III. The Prosencephalic or frontal arch is based on the 

 pre-sphenoid already described in connection with the 

 basi-sphenoid. The orbi/o-spfieiwids or neurapophyses of 

 the fiontal vertebra are usually square, but sometimes semi- 

 circular or semi-elliptic, as in the Cod ; larger in the Mala- 

 copteri, but very small usually in the Aranthopteri, and 

 cartilaginous in the Sclerogenidip. The olfactory nerves 

 pass out of the skull by the superior interspace of the 

 orbito-sphenoids, and the optic nerves by their inferior 

 interspace or by a direct ])erforation. The mid-frontnl 

 bone completes this arch above, and alwavs enters into the 

 formation of the cranial cavity though its major part forms 

 the roof of the orbits. It is single, and sends up a median 

 crest in the Cod, the Ephippus, and some other fishes, but 

 is more commonly divided along the median line. Each of 

 the halves sends up its own crest in the Tunny. In the 

 Salamandroid fishes, each frontal sends down a vertical 

 plate to the sphenoid, thus forming a canal through which 

 the olfactory nerves run. The side walls of this canal 

 make a double bony partition between the orbits. The 

 post-froH/als or parapophyses of the fi-ontal vertebra partly 

 underlie the mastoids, and complete their prominent crest. 



IV. The Rhinencephalic arch terminates the axis of the 

 skull anteriorly, and protects the olfactory ganglions and 

 nerves. The centrum of the nasal vertebra is named the 

 vomer. Its posterior point is wedged into the under part 

 of the pre-sphenoid ; it expands and becomes thicker, an- 

 teriorly, where its lateral angles are articulated to the pre- 

 frontals. Its ujiper surface supports the nasal bone, and 

 its under surface, which enters into the composition of the 

 roof of the mouth, is in many fishes dentiferous. The 

 same is of\en the case 'with the palatine bones, which abut 

 against it laterally, and also form jiart of the roof of the 

 mouth. The presence or absence of these teeth has been 

 made much use of for characterizing genera and large 

 groups of the osseous fishes. In Lepidostens the vomer is 

 cleft by a median fissure. In Poli/pferus it is confluent 

 with the basi-sphenoid. The pre-frontah or neurapo- 

 ])hyses of the nasal vertebra defend and support the ol- 

 factory ganglions, bound the orbits anteriorly, give a surface 

 of suspension to the palatine bones, and through them for 

 the palato-maxillary arch ; and they also give attachment 

 to the large pre-orbital or lachrymal scale bone when this 



exists. In the Cyprinoids and most Clupeoids the pre- Introduc- 

 fi-ontals form ])art of the interobital septum. In the Con- t>on. 

 gtrid(B we recognise the pre-frontals in the confluent parts Osteology, 

 of the nasal vertebra bv the external jn-oove conducting ^■^"V"^ 

 the olfactory nerves to the nasal capsules, and by the in- 

 ferior process from which the palatine bone is suspended. 

 In the Murtfinda also the pre-frontals are plainly confluent 

 with the nasal bone, and furnish the well-marked articular 

 surfaces for the palato-maxillary bone. In the Cod the 

 palatine arch is chiefly, but not whoUv, suspended to the 

 ])re-fi-ontals. The iiusal-bone or spine of the most anterior 

 arch of the skull is usually single, and terminates forwards 

 in a thick obtuse extremity. In the Salnionoids and some 

 other fishes, it is broad, but not deep ; in Istiopliortts it is 

 long and narrow ; in the Discoboles and Lopliohranchii it 

 is a short vertical compressed plate, and it is wanting in Lo- 

 phiits and Dioilon, or merely membranous. It is articulated 

 above and behind to the fiontals and pre-frontals ; and be- 

 low, either directly or by cartilage, to the vomer. In the 

 Flving Gurnard it has no immediate connection with the 

 vomer ; but this is a rare exception. In the Salamandroid 

 fishes, the nasal is divided by a median suture. The horn- 

 like projection from the forehead of Naseus is formed 



Fig. Ifi. 

 Naseits lon<jiconiis, one of tbe TltexiiyidT. 



chiefly from the frontal bone, a small nasal being articu- 

 lated beneath. This nasal is trifid anteriorly ; its lateral 

 divisions articulating with the pre-maxillaries, as in Citha- 

 rinus. The turbhuil bone or osseous capsule of the nose, 

 belonging to the splanchno-skeleton, is situated at the side 

 of, or above the nasal ; the pre-maxillary and maxillary 

 bones are usually attached to its extremity through the 

 medium of cartilage. In the Murcenidte, the normal ele- 

 ments of the nasal vertebra coalesce into a single bone. 

 The spine of this vertebra or nasal bone forms the usual 

 obtuse extremity beneath the skin of the upper part of 

 the snout, and it is dentiferous, as in the Lepidosiren; it is 

 intimately confluent anteriorly with the centrum or vomer, 

 the limits being indicated by the interruption of the median 

 series of vomerine and nasal teeth. 



This completes a compendious view of the neural arches 

 of the four cranial vertebra? which have fiir their primary 

 function the protection of the ganglionic brain, in which 

 the spinal chord ends anteriorly. From the complete os- 

 seous skull of a fish, there are many descending modifica- 

 tions, until we come to the vermiform Lancelet, which wants 

 the cerebral expansion altogether ; the gelatinous " chorda 

 dorsalis" being terminated by a tapering end, invested by a 

 membrane, without any addition of cartilaginous or osseous 

 coverings. 



p.. Sense capsules of the s]ilanchno-skeleton intercalated 

 with the neural arches. Some of these have been already 

 noticed. In the Ammocutes, cartilage is developed for the 

 protection of the acoustic organ, in a skull, otherwise 

 mostly fibro-membranous. (a.) The bones entering into 

 the formation of the otocrane for the lodgement of the 

 labyrinth have been named above ; but it is the petrosal 

 which is the bone specially designed to form the acoustic 

 capsule, though in fishes it is less confined to that function 

 than in the higher vertebrals in which it exclusively enve- 

 lopes the labyrinth. Its ossification commences late in the 

 osseous fishes. It is very small in the Cyprinoids, and in 

 them Cuvier describes it as a d.smeniberraent of the 



