vin SKULL 2O9 



various parts of the head. In many Elasmobranchs the roots 

 of certain of the anterior spinal nerves perforate the side-walls 

 of the occipital region, and indicate the fusion of vertebral 

 components with the cranium. In the cranial roof between the 

 two periotic capsules there are two small apertures at the bottom 

 of a common median depression : through each aperture the 

 ductus endolymphaticus (aqueductus vestibuli') passes from the 

 vestibular part of the auditory organ to the exterior of the- skull. 



Three cartilaginous rods, one from the roof of each olfactory 

 capsule, and one, the prenasal or rostral process, from the ethmoid 

 cartilage, converge and meet, or nearly meet, in front to form 

 the rostrum or support for the preoral or " cut-water " portion 

 of the head. 



The visceral arches are seven in number. The first or man- 

 dibular arch consists on each side of an upper portion, the palato- 

 ptery 'go-quadrate or palato-quadrate cartilage, which passes forwards 

 in the side-wall of the oral cavity, along the upper margin of the 

 mouth, its anterior or palatine part curving inwards to a liga- 

 mentous connexion with its fellow beneath the cranial floor. 

 Each cartilage has an upwardly directed process (ethmo-palatine 

 process) which is connected bya suspensory ethmo-palatine ligament 

 with the lateral wall of the cranium behind the lateral ethmoid 

 process. The lower or ventral half of the mandibular arch 

 (Afeckel' s cartilage) is similar in shape to the upper ; it articu- 

 lates behind with the quadrate portion of the latter by a movable 

 joint, and is thence prolonged forwards and downwards in relation 

 with the lower margin of the mouth to a median ligamentous 

 union with its fellow of the opposite side. The palato-pterygo- 

 quadrate and Meckel's cartilages together form the primitive 

 upper and lower jaws, and support the teeth. The hyoid arch 

 also consists of a dorsal and a ventral half on each side. The 

 dorsal half or hyomandibular element articulates above with the 

 periotic capsule. The ventral portion, or cerato-hyal, passes 

 downwards and is connected with its fellow by a median copula 

 or Itasi-hyal cartilage situated in the floor of the oral cavity. A 

 series of simple cartilaginous rays (branchial rays) are attached 

 to the hinder margins of the hyomandibular and cerato-hyal 

 elements. The distal end of the hyomandibular is connected 

 by strong ligaments with the hinder portions of both the 

 palato-pterygo-quadrate cartilage and Meckel's cartilage ; in fact, 



VOL. VII P 



