700 MR. PARKER ON THE SKULL IN SHARKS AND RAYS. [ NoT. 7, 



branchial arches that give gill- plates to the back of one space and to» 

 the front of the next these rudiments are arranged in a double row, 

 and look like the cogs of a wheel. The foremost arch with permanent 

 gill-folds is the hyoid ; and this, like the last branchial arch, can 

 have but one series of plaits. 



" The mouth of a Selachian is much modified ; the mandibular 

 bar, before it is segmented into the suspensorium and free mandible 

 (Meckel's cartilage), grows not upwards to the auditory region, but 

 forwards to the back of the nasal sacs. 



" Then a joint is formed, and a knob on the upper piece fits into a 

 hollow on the lower ; the upper piece is the quadrato- palatine 

 arcade or upper jaw, and, like the lower jaw, is articulated to its 

 fellow of the opposite side by ligamentous substance. 



" There is a free cartilage above the quadrate in the Skate, the 

 1 spiracular cartilage,' which is the proper, but detached, apex of the 

 suspensorium. In the Lesser Spotted Dogfish there is nothing but a 

 ligament ascending from the quadrate ; but in some of the Sharks 

 there is a small ray. in others two or even three of these rays, which 

 are largely developed in the hyoid and branchial arches, forming the 

 skeleton of the interbranchial folds. 



" These folds are made still more strong in the Sharks by external 

 cartilages that run outside each septum ; these ' extrabranchials ' 

 are not developed in the Skates. 



" In both groups there is a complex system of 'labial cartilages/ 

 helping to form the ' rostrum,' and to supply valves for the nasal 

 openings ; in the Sharks the ftps also have two or three pairs of these 

 4 extraviscerals.' 



" The three pairs of sense-capsules have tracts of cartilage between 

 them ; and these may be called the ' intercapsular ' bands generally : 

 the inter uuditonj are the parachordals, the interocular the 'tra- 

 becule,' and the internasal are the nasal septum and trabecular 

 cranii. These latter grow into the face as a rudimentary visceral 

 arch ; it is composed of a pair of lateral processes and the azygous 

 prenasal rostrum— the axis of the cutwater, so large in the Skate and 

 Saw-fish. 



" On each side of the nasal sac in Skates and some Sharks there 

 is another pair of visceral arches, the eth mo-palatines. These are 

 distinct cartilages ; in some Sharks, as in Scyltium cunicula, they 

 exist as exogenous rudiments. 



" As a rule there are five clefts or facial slits besides the spiracle ; 

 Hexanchus and Ileptanchas have more, as their name implies. 



" One or two more interesting facts may be mentioued : the noto- 

 chord acquires a cartilaginous sheath of its own, and in young 

 embryos it is beaded in front ; the ' investing mass,' or interaudi- 

 tory cartilaginous bands, runs on undivided far into the vertebral 

 region." 



This paper will be printed entire in the Society's ' Transactions.' 



Eeferring to Canon Tristram's recent " Note on the Discovery of 



