SKELETON OF KEGALECUS AEGENTEUS. 15 



The upper jaw is completed, as usual, by two pairs of membrane bone, the pre- 

 maxillse and the maxillae. 



The premaxilla (fig. 6, pmx) is a slender bone consisting of two well-marked parts, 

 an alveolar portion {fmx) which bounds the gap, and consists of a thin narrow plate of 

 bone, strengthened by a raised rib along its outer surface ; and a nasal process {jpmx^) 

 which passes backwards and slightly upwards, parallel to and in contact with its 

 fellow of the opposite side, and embracing splint-wise the dorsal edge of a large 

 laterally compressed cartilage. This latter works freely in the groove of the tegmen 

 cranii and between the anterior processes of the frontals, the protrusion and retraction 

 of the jaws being thus provided for. 



This cartilage is evidently homologous with the irregular nodule which supports the 

 premaxillse in the Cod ( Gadus), and with which every one who has dissected the head 

 of that fish must be familiar. Curiously enough, no mention is made of it by either 

 Stannius, Owen, Huxley, Gegenbaur, Macalister, or Parker and Bettany. 



The maxilla (fig. 6, mx) is a widish plate of bone, narrowing gradually from its lower 

 to its upper extremity, and produced at its upper end into a strong internal process. It 

 does not enter into the gap, and its whole outer surface is subcutaneous (see fig. 1). 



The form of the lower jaw is remarkable. It consists of two rami, loosely united by 

 fibrous tissue, each of which may be described as consisting of a suprameckelian and 

 an inframeckelian portion, separated from one another by Meckel's cartilage (fig. 14, 

 7)ic7i}. The suprameckelian portion has something the form of an equilateral triangle, 

 the inframeckelian of a right-angled triangle with altitude about one fourth of its 

 base, so that the whole jaw comes to be rather higher than long. Each ramus contains 

 the usual three bones. 



The articular (figs. 6 and 14, ar) is a large bone furnishing a concave facet for 

 articulation with the quadrate ; it forms the posterior half of the suprameckelian 

 portion of the jaw, and about the posterior sixth of its inframeckelian portion. From 

 near the proximal end of its inner surface a slender Meckel s cartilage {mck) is 

 continued to the symphysis. 



The angular {ang) is a small rod-like bone, applied to the proximal end of the inner 

 surface of the inframeckelian portion of the articular. 



The dentary (d) is a considerable bone, forming the anterior half of the supra- 

 meckelian, and the anterior five sixths of the inframeckelian portions of the mandible. 

 It is united to its fellow of the other ramus by fibrous tissue forming the mandibular 

 symphysis. 



3. The Opeecul.\r Bones. (Plate IV. fig. 6, Plate V. fig. 14.) 

 The number and disposition of these bones is quite normal ; like the other subcu- 

 taneous bones, they are sculptured externally, and covered in the recent state by an 

 extremely thin layer of silvery epiderm {cf. fig. 1). 



