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Show 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 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 ' 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 ' extrabrauchials' 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 lips also have two or three pairs of these ' extraviscerals.' " The three pairs of sense-capsules have tracts of cartilage between them ; and these may be called the ' intercapsular' bands generally : the interauditory are the parachordals, the interocular the ' trabeculae,' and the internasal are the nasal septum and trabeculae 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 ethmo-palatines. These are distinct cartilages; in some Sharks, as in Scyllium canicula, they exist as exogenous rudiments. " As a rule there are five clefts or facial slits besides the spiracle ; Hexanchus and Heptanchus have more, as their name implies. " One or two more interesting facts may be mentioned : the notochord acquires a cartilaginous sheath of its own, and in young embryos it is beaded in front ; the ' investing mass,' or interauditory cartilaginous bands, runs on undivided far into the vertebral region." This paper will be printed entire in the Society's 'Transactions.' Referring to Canon Tristram's recent " Note on the Discovery of |
