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.
(c) Each of the five branchial arches is a hoop, incomplete above and formed of four or more pieces of cartilage. The most dorsal elements, the pharyngo-branchials, are flattened, pointed plates whose free inner ends run obliquely backwards, and terminate below the vertebral column. They are connected at their outer ends with the short broad epi-branchials (fig. 6, 16) which lie at the sides of the pharynx. From the epi-branchials arise the long cerato-branchials (fig. 6, 17) which run forwards and inwards along the ventral wall of the pharynx. The first four cerato-branchials are connected with small rods, the hypo-branchials, which run backwards to meet one another in the middle line. The last two pairs of hypo-branchials and the fifth cerato-branchials are connected with a broad median plate, the basibranchial. Along the outer sides of the second, third and fourth cerato-branchials are found elongated curved rods, the extra-branchials (fig. 6, 19). The epi-branchials and cerato-branchials bear gill rays along their posterior borders.
C. The Skeleton of the Median Fins.
The dorsal fins have a skeleton consisting of a series of short cartilaginous rods, the basals or basalia, which slope obliquely backwards. Their bases are imbedded in the muscles of the back, while their free ends bear a number of small polygonal cartilaginous plates, the radials or radiale. Associated with this cartilaginous skeleton are a number of long slender horny fibres, the fin-rays, which have been already referred to in connection with the exoskeleton. The skeleton of the other median fins mainly consists of these fibres, the cartilaginous portion being reduced or absent.
2. The Appendicular Skeleton.
This includes the skeleton of the two pairs of limbs and of their respective girdles.
The Pectoral girdle forms a crescent-shaped hoop of cartilage, incomplete above and lying just behind the visceral skeleton. The mid-ventral part of the hoop is the thinnest portion, and is drawn out in front into a short rounded process which is cupped dorsally and supports part of the floor of the pericardium (fig. 7, 1). On each side of this flattened mid-ventral portion the arch becomes very thick and bears on its outer border a surface with which the three basal cartilages of the fin articulate. The dorsal ends or scapular portions of the girdle form a pair of gradually tapering horns.
The Pectoral fin articulates with the pectoral girdle by means of three basalia or basal cartilages, the propterygium, meso-pterygium and meta-pterygium. The most anterior and the smallest of these is the propterygium (fig. 7, 5), while the most posterior one, the meta-pterygium (fig. 7, 3), is much the largest. Along the outer borders of the three basalia are arranged a series of close set cartilaginous pieces, the radiale. The propterygium supports only a single radial, which is however much larger than any of the others. The meso-pterygium also supports only a single radial which divides distally.
