STRUCTURE OF FISHES. 



it does not often happen that the temporal muscles, which work the lower jaw, extend to 

 the upper surface of the cranium, though this sometimes occurs, as in the Conger Eel and in 

 the Lepidosiren. The bones of the skull, as in man, are divided into those of the brain-case and those 

 of the face, but in fishes the facial bones are far more developed than in man. The hindermost 

 bone at the back of the skull, by which it joins the first vertebra, varies a good deal in form. In 

 most fishes, as in the Carp, it is a deep conical cup, but in the Holibut it is almost flat, and in 

 Fistularia it presents a convex surface, which is exactly comparable with the condition seen in a 

 Lizard or Crocodile. This bone, which is termed the basi-occipital, in many fishes has a process 

 prolonged downward from its under side, and in the Carp this broad triangular plate supports the 

 large upper grind ing-tooth in the throat, reminding one of the way in which processes from the 

 neck vertebrae are prolonged into the oesophagus in certain Snakes and Lizards. There are two 

 bones rising from the basi-occipital which arch over the beginning of the brain ; they are termed the 

 ex-occipitals. They are usually perforated for the pneumo-gastric nerve, and sometimes for other 

 nerves also. The bone above these is called the supra-occipital. In the Cod it is prolonged back- 

 ward in a median spine ; in the bony Pike (Lepidosteus) it is double, being divided by a suture 

 in the middle line. Sometimes the crest of the bone is exceedingly lofty, as in the Light Horseman 

 fish (Ephippus), and sometimes absent, as in the sucking fish Remora. At the sides of the ex-occipitals 

 are two bones, termed by Owen par-occipitals, and by Huxley opisthotic bones. The distinction of these 

 bones in the lower Vertebrates is a characteristic feature of the skull, but in the Polypterus they 

 unite with the ex-occipital bones, 



as in Batrachian reptiles, and in - " -^-^ s 



the Chad they unite with the 

 mastoid bones, as in the Chelonia. 

 The organ of hearing in fishes is 

 usually large. In front of this 

 girdle of cranial bones, which 

 Sir E. Owen long ago com- 

 pared to one of the trunk ver- 

 tebrae, is a similar series of bones 

 with a median basal bone, now 

 called the para-sphenoid, and 

 closely resembling the para- 

 sphenoid of Amphibians, which 

 reaches along the greater part of 

 the base of the skull, exactly as 

 in that group. In the flat fishes 

 its anterior end is twisted upon 

 one side of the skull. It is 

 always smooth below, and in the 

 genus Polypterus the bones which 

 rise from its sides are blended with 



it. There is some difference of opinion with regard to the names to be given to these bones. The 

 hindermost bone is termed by Owen ali-sphenoid, and by Huxley prootic. These bones are arched 

 over by the parietals. In the Salmon family the two parietals soon unite together. In the Siluroid 

 fishes they also unite with the supra- occipital bone, but in many osseous fishes the supra-occipital 

 bone extends between them. They are always flattened above. In some fishes they are perforated 

 by nerves which supply the vertical fins of the back. In front of the parietal bones is the principal 

 frontal, which roofs over the orbits in all animals. It carries a median crest in the Cod and some 

 fishes, and varies in shape with the form of the skull. In the Tunny each frontal has a crest of its 

 own, and in some Siluroid fishes and the Loach there is a fontenelle between the frontal and parietal 

 bones which corresponds in position with the so-called foramen parietale, which is characteristic of 

 many fossil and living reptiles. In the flat fishes the frontal is single. Behind it at the sides are the 

 post-frontal bones, which assist in arching over the auditory cavities, and help to furnish a support for 



SKULL OF CODFISH (Gadus morrhua). [After Otoeii.] 



I, Lacrymal; n, Nasal; /.Frontal; s, Surra-occipital ; pm, Pre-maxillary ; m^ Maxillary ; to., 

 Inter-operculura; c.o., Sulvoj 

 g. Quadrate; c.h., Cerato-liya 



FJVBWI . *< oui'i it-vn . ii'iutii , fmt i I t-iimAt nni .) , lit, .jiaAjuni j , -i/.| 



Int'ef^opercuiiini; c.o., Sub-opercuHim; t>. OpercOlum : p.o. PVe-oi>erculum: h.m.. Hyo-mandibular; 

 ' al; 6, BrauchiosU'gal Kays; ps, Para-sphenoid ; d t Dentary. 



