84 LECTURE IV. 



spines ; a post-frontal extends, like a bony scale, above the roof of 

 the cranium, and over the strong temporal muscles, which are at- 

 tached to the inner surface of that bone, as to an exo-skeleton ; and 

 one or two opercular bones are superadded behind the simple pe- 

 dicle of the jaw. But the single concave surface presented by the 

 basi-occipital to the vertebriE of the trunk, the lower transverse pro- 

 cesses {parapophyses) of the abdominal vertebrae, and the articulation 

 of the scapulo-coracoid arch to its proper ei'anial vertebra, afford 

 unequivocal evidence that the Lepidosiren is a true Fish. 



LECTURE V. 



THE SKULL OF OSSEOUS FISHES. 



Having noticed the principal facts in the development of the skull in 

 the embryo of an Osseous Fish, and the several stages at which that 

 development is arrested, or diverted to acquire special modifications, 

 in the Cartilaginous Fishes ; and having dwelt more particularly on 

 the instructive semiosseous, semicartilaginous skull of the Lepi- 

 dosiren, I proceed, in the present Lecture, to the demonstration of 

 the complex skull of the Osseous Fishes, which constitute the great 

 bulk and the typical members of the class ; and, for that purpose, I 

 shall take one of our largest and most common species — the Cod-fish 

 (Gadus Morrhua) — in which you may easily repeat the observations 

 and test the conclusions about to be submitted to you. In describing 

 the general form and composition of this skull, according to my views 

 of the homologies of its constituent bones, I shall also indicate the 

 most instructive and remarkable modifications of the skull in other 

 Osseous Fishes, and notice those which, stopping short of the acqui- 

 sition of the most characteristic features of the fish's skull, lead more 

 directly to the cranial type of higher Vertebrata. 



The head is larger in proportion to the trunk in Fishes than in 

 any other class of animals ; it forms a cone whose base is vertical, 

 directed backwards and joined to the trunk without an intervening 

 neck, and the sides three in number, one superior and tAvo lateral and 

 converging downwards : the cone is shorter or longer, more or less 

 compressed, more or less depressed, with a sharper or blunter apex, in 

 different species. The base of the skull is perforated by the foramen 



