86 



vertebras: the skull here meaus the bony structure which coiitaius the braii>, and 

 supports the eyes, ears, and olfactory organs, and which remains as a united whole 

 when the soft parts of the head of the fish are removed by boiling or otherwise. The 

 form of the skull is shown by four drawings of it on Plate XI (Figs. 5-8). It consists of 

 two main portiou.s. The posterior and larger portion has a somewhat cylindrical shape, 

 and encloses a cavity m which the brain lies in the entire fish, this is the cranial 

 portion ; the anterior or facial portion does not enclose a cavity, but may be said to 

 consist of three separate processes from the brain-case which unite together at their 

 anterior ends. One of these processes is (apparently) on the right side of the head of the 

 fish and separates the eyes from one another ; it may be called the interorbital septum, 

 the not very definite concavities on either side of it in which the eyes lie being the 

 orbits. 



If the skuU is placed in the position it occupies when the entire fish is held vertically 

 upright on its ventral edge, that is in the same position in which a symmetrical fish 

 swims, the cranial portion of the skull will be seen to be almost perfectly symmetrical 

 while no symmetrj- is visible in the facial portion. Comjjaring the cranial portion 

 with the corresponding part of the skuU of a symmetrical fish, we can recognise in its 

 walls the same bones as in the latter, the walls being made up of these bones firmly 

 united at their edges. These edges are usually irregularly toothed so as to fit into 

 each other, and the lines of junction are called sutures. The sutures can be traced out 

 with care in the entire cranium, Init where they are obscure the exact limits of the 

 component bones can be found by boiling the skull and pulling the separate bones away 

 from one another. The outlines of these bones are indicated on the figures of the 

 skull. Tlie auditory organs in the entire fish are situated within the convex side walls 

 of the cranial portion, and the bones of this part have names with the suffix "otic," 

 implying their relation to the ear. 



The back of the cranial portion is called in all vertebrates the occiput and the bones 

 of this part occipital bones. In this wall of the cranium is a large circular aperture, 

 the occipital foramen, through which the spinal cord is continued forwards into the 

 brain. Below the occipital foramen is a solid bone witli a conical depression in its 

 posterior surface, this is the basi-occipital, and the depression marks the place where 

 the first vertebra is attached to the skull. At the sides of the occipital foramen 

 inferiorly are the two ex-occipital bones, ex.o., one on each side. Above these, reaching 

 to the upper edge of the foramen, are two large somewhat convex bones, the epiotics, ep.o. 

 On the dorsal surface of the skull there is in the centre a large median flat bone, the 

 supra-occipital, s.o., whicli in many vertebrate skulls extends to the edge of the occipital 

 foramen, but is here separated from it by the epiotics. On either side of the 

 supra-occipital is a parietal bone, pa. In either lateral wall of the cranium, adjoining 

 the edges of the ex-occipital, epiotic, and parietal, there is a somewhat large Ijone 

 having a rugged process projecting outwards ; this is the pterottc bone, pt.o. In front 

 of this is a shghtly smaller square bone, the sphenotic, sp.o. Between the pterotic 



