THE ELASMOBRANCH FISHES 211 



The cranium of the dogfish is made up of a brain-case to 

 which two pairs of capsules of the organs of special sense are 

 fused. As no bone enters into its composition, and it has no 

 covering of bony plates, it is a particularly favourable object 

 for the study of the primitive condition of the vertebrate skull. 

 The brain-case proper is a somewhat oblong cartilaginous box 

 open behind and in front. The hinder opening, situated on 

 the posterior wall of the cranium, is the foramen magnum, 

 through which the spinal cord passes to expand into the brain. 

 On the sides of the foramen magnum are the two occipital 

 condyles, for articulation with the first vertebra. The opening 

 in front is a large vacuity in the cranial roof. During life it is 

 covered in by a sheet of connective tissue, with which the 

 pineal body enters into close relations. The olfactory capsules 

 are fused to the front end of the cranium, and in the adult are 

 continuous with it, but in embryonic life they were separate. 

 They are large hemispherical sacs with wide openings looking 

 downwards and forwards. Their walls are very thin, and their 

 cavities are separated by a median vertical cartilaginous 

 plate, the internasal septum. Three small cartilaginous rods 

 project forward in front of the olfactory capsules to form the 

 rostrum. The two uppermost spring from the walls of the 

 olfactory capsules, the lowest from the front edge of the 

 internasal septum, and they converge together in front. In 

 Scyllium they are very small, but in some sharks and dogfishes 

 they are relatively large. 



The auditory capsules, like the olfactory, were separate in 

 embryonic life, but in the adult they are intimately fused with 

 the side walls of the hinder part of the cranium, and form a 

 pair of prominent projections immediately behind the orbits. 

 A ridge running forward and outward on the roof of each 

 capsule, marks the position of the anterior vertical semicircular 

 canal ; a similar ridge running backward marks the posterior 

 vertical semicircular canal, and a horizontal shelf marks the 

 external horizontal canal. Between the two auditory capsules, 

 on the mid-dorsal roof of the cranium, is a depression in 

 the sides of which are the openings of the ductus endo- 

 lymphatici or aqueductus vestibuli leading into the auditory 

 organs. 



The optic capsules are never fused to the cranium in any 

 vertebrate, but they profoundly modify the walls to which they 



