THE ELASMOBRANCH FISHES 213 



The upper figure represents the skull, visceral skeleton, the anterior part of the 

 vertebral column and the pectoral girdle and iin of Scy Ilium canicula. The 

 lower figure shows the skeleton of the first doisal fin attached to a section of ihe 

 vertebral column ; the last few vertebra; have been cut through vertically. In the 

 skull au, auditory capsule ; ol, olfactory capsule ; re, rostrum ;/-, palate-quadrate 

 cartilage; md, mandibular cartilage; /, post -spiracular ligament; //, optic 

 foramen ; ///, foramen for 3rd nerve ; IV, foramen for 4th nerve ; V t foramen for 

 main branch of sth, 6th, and main branch of yth nerve ; Fa, F//a, foramina for oph- 

 thalmic divisions of the sth and yth nerves; h, foramen for hyoidean artery; ic, 

 interorbital canal. In the vertebral column v, vertebral plate ; iv, intervertebral 

 plate ; jii, foramen for dorsal root, and J"v, foramen for ventral root of a spinal 

 nerve. In the visceral skeleton hyi, hyomandibular cartilage ; bh, basihyal ; ch, 

 ceratohyal \pbr, pharyngobranchials ; cbr, epibranchials ; cbr, ceratobranchials ; hbr, 

 , hypobranchials ; the gill-rays are shown on the epi and ceratobranchials and on the 

 hyomandibular and ceratohyal cartilages. The extra branchial cartilages are 

 omitted. In the pectoral girdle sc, scapula ; cor, coracoid ; ppt, propterygium ; 

 incs.pt) mesopterygium ; m.pt, metapterygium ; act, actinotrichia. In the lower 

 figure b, basals ; ra, radials ; act, actinotrichia ; na, neural arches ; nt, spinal 

 cord ; nek, the notochord constricted intravertebrally. 



are contiguous, and are lodged in spacious orbital cavities. 

 The orbital cavities of the dogfish are bounded by the olfactory 

 capsules in front, the auditory capsules behind; the roof is 

 formed by a supra-orbital ridge of cartilage projecting from the 

 cranial roof, and the floor below is formed by a horizontal plate 

 of cartilage continuous with the cranial floor. Between each 

 supra-orbital ridge and the ridge of the cranial roof there is a 

 wide shallow groove in which a bundle of mucous canals is 

 lodged, and at the anterior end of the groove is a foramen 

 through which the ophthalmic branches of the fifth and seventh 

 nerves pass from the orbit to be distributed over the upper 

 surface of the snout. The inner wall of the orbit is perforated 

 by the following apertures or foramina, whose positions can 

 easily be studied in fig. 52: The optic foramen, through 

 which the optic nerve passes from the brain to the orbit ; the 

 foramen for the third nerve ; the foramen for the fourth nerve ; 

 the foramen for the main branches of the fifth and seventh and 

 the sixth nerves, a large hole in the posterior corner of the 

 orbit ; the foramen for the ophthalmic branches of the fifth 

 and seventh nerves lying close together above and rather in 

 front of, the last named foramen. The aperture of the inter- 

 orbital canal lies in front of and rather below the foramen 

 for the fifth and seventh nerves, and admits the passage of 

 a blood-vessel putting the venous sinus of the orbit of one 

 side into communication with that of the other. Posteriorly 

 there is a deep groove overhung by the projecting ridge of 

 the horizontal semicircular canal, and leading from the orbit 

 to the back of the skull. In life it admits the passage of 



