222 MR. A. S. WOODWARD ON THE MANDIBULAR [Apr. 20, 



is much compressed from side to side, the superior edge thus formed 

 being well marked and sharp, but the lower appearing thicker 

 (fig. 2 a). About two thirds of the distance from the upper extremity 

 a gradual twist in the cartilage renders the remaining portion com- 

 pressed almost from above downwards, and it ends distally in a 

 triangular expansion, well shown on the right. The terminal edge 

 was evidently articulated to a basihyal of considerable size, but of 

 this no trace has been preserved. 



At one third of its length from the proximal end, the cerato-hyal 

 exhibits a prominent tuberosity on the lower border (fig. 2, tu). 

 This appears to be situated opposite a point corresponding to the 

 attachment of the mandibulo-hyoid ligament of living Selachians ' ; 

 but I have failed to discover indications of any similar prominence either 

 in Heptanchus or the other specimens and figures to which I have 

 been able to refer. It evidently represents a muscular insertion, and 

 one of no small importance. In Teleostean fishes, the well-developed 

 genio-hyoideus arises from an equivalent point in the hyoid arch, and 

 by its insertion at the symphysis acts as the main depressor of the 

 mandible ". In the Selachii, however, the function is undertaken 

 chiefly by the coraco-mandibulares arising from the pectoral arch, 

 and a differentiated genio-hyoid appears to be wanting. The elabo- 

 rate researches of Vetter ^ in regard to the myology of the jaws and 

 branchial arches in Heptanchus and Acanthias can leave no doubt 

 that the tuberosity in question is that deep insertion of the great 

 constrictor superficialis muscle which becomes the origin of the 

 genio-hyoid in higher fishes ; and its marked character in the Creta- 

 ceous Hybodont may perliaps indicate that in this form the differen- 

 tiation had already taken place to a certain extent. 



On the whole, the form of hyoid arch just described bears a 

 greater resemblance to that of the NotidanidcC than to that of any 

 other living family. It agrees in the fact tliat the hyomandibular 

 and cerato-hyal are most contracted at their point of union, but the 

 elements are somewhat stouter than those both of Heptanchus and 

 Hexanchus. 



The tyj)e specimen of Flybodws dubrisiensis (B.M. 36908) is of 

 somewhat smaller size than the fossil here described, and as its 

 cartilages are apparently less calcified, it probably represents a 

 younger individual. It is comparatively imperfect, and except in 

 the characters of the teeth and the upper border of the pterygo- 

 quadrate, it does not admit of any detailed comparison with the 

 foregoing descriptive account. There are, however, unmistakable 

 indications of an articular otic process, besides a well-marked pterygo- 

 trabecular eminence ; and a small fracture shows the slight thickening 

 of tlie overturned edge of the quadrate region (fig. 6). 



^ W. K. Parker, "On the Structure and Development of the Skull in Sliarks 

 and Skates," Trans. Zool. Soc. vol. x. p. 210, pi. xxwiii. fig. 2. 



- R. Owen, ' Anatomy of Vertebrates,' vol. i. p. 2()6, fig. 1.3."). 



^ B. Vetter, " Untersuchuugen ziir vergleichenden Anatomie der Kieinen- 

 und Kiefermusculatur der Fiscbe. — I ,' Jeuaiscbe Zeitachrift, vol. vii. (1874), 

 pp. 403-458, pis. xiv., xv. 



