(_a_) The first or =mandibular arch= is the largest of the series, and forms the upper and lower jaws. Each half of the upper jaw or =palato-pterygo-quadrate= bar is formed by a thick cartilaginous rod which meets its fellow in the middle line in front, the two being united by ligament. Each half is connected to the cranium just in front of the orbit by the =ethmo-palatine ligament= (fig. 6, 10), and at its hind end articulates with one of the halves of the lower jaw. Each half of the lower jaw or =Meckel's cartilage= (fig. 6, 12) is a cartilaginous bar, wide behind but narrow in front, where it is united to its fellow by a median ligament. Imbedded in the tissue external to the upper jaw are a pair of =labial cartilages=, and a similar but smaller pair are imbedded in the tissue external to the lower jaw. The jaws are developed from a structure whose dorsal and ventral portions subsequently become of very different importance. The ventral portion forms both upper and lower jaws, the former being developed as an outgrowth from the latter. The dorsal portion forms only the =pre-spiracular ligament= (fig. 6, 20), a strong fibrous band containing a nodule of cartilage, and running from the anterior part of the auditory capsule to the point where the jaws are connected with the hyomandibular. (_b_) The =hyoid arch= consists of a pair of cartilaginous rods which are attached at their dorsal ends to the cranium, and are united ventrally by a broad median plate of cartilage, the =basi-hyal=. Each rod is divided into a dorsal portion, the =hyomandibular= and a ventral portion, the =cerato-hyal=. The =hyomandibular= (fig. 6, 13) is a short stout rod of cartilage projecting outwards, and somewhat backwards and downwards from the cranium, with which it articulates behind the orbit and below the postorbital groove. Its distal end articulates with a rather long slender bar, the =cerato-hyal= (fig. 6, 14), which is in its turn attached to the side of the =basi-hyal=. The =basi-hyal= is a broad plate, rounded in front and drawn out behind into two processes to which the two halves of the first branchial arch are attached. The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, the =gill rays=. The hyoid arch forms the main =suspensorium= or means by which the jaws are attached to the cranium. This attachment is chiefly brought about by a series of short ligaments which connect the posterior ends of both upper and lower jaws with the hyomandibular, but there is also a ligament connecting the lower jaw with the cerato-hyal. The attachment of the jaws to the cranium is also partially effected by the pre-spiracular and ethmo-palatine ligaments.