The upper and lower jaws become gradually prolonged, till they eventually form a snout; while at the end of the upper jaw is placed the suctorial disc, which is now considerably reduced in size (fig. 68, _sd_). The "fleshy globular termination of the upper jaw of the adult Lepidosteus is the remnant of this embryonic sucking disc." (Agassiz, No. 92.) The fin-rays become formed as in Teleostei, and parts of the continuous embryonic fin gradually undergo atrophy. The dorsal limb of the embryonic tail, as has been shewn by Wilder, is absorbed in precisely the same manner as in Teleostei, leaving the ventral lobe to form the whole of the permanent tail-fin. BIBLIOGRAPHY. (92) Al. Agassiz. "The development of Lepidosteus." _Proc. Amer. Acad. of Arts and Sciences_, Vol. XIII. 1878. _General observations on the Embryology of the Ganoids._ The very heterogeneous character of the Ganoid group is clearly shewn both in its embryology and its anatomy. The two known types of formation of the central nervous system are exemplified in the two species which have been studied, and these two species, though in accord in having a holoblastic segmentation, yet differ in other important features of development, such as the position of the yolk etc. Both types exhibit Teleostean affinities in the character of the pronephros; but as might have been anticipated Lepidosteus presents in the origin of the nervous system, the relations of the hypoblast, and other characters, closer approximations to the Teleostei than does Acipenser. There are no very prominent Amphibian characters in the development of either type, other than a general similarity in the segmentation and formation of the layers. In the young of Polypterus an interesting amphibian and dipnoid character is found in the presence of a pair of true external gills covered by epiblast. These gills are attached at the hinder end of the operculum, and receive their blood from the hyoid arterial arch[41]. In the peculiar suctorial disc of Lepidosteus, and in the more or less similar structure in the Sturgeon, these fishes retain, I believe, a very primitive vertebrate organ, which has disappeared in the adult state of almost all the Vertebrata; but it is probable that further investigations will shew that the Teleostei, and especially the Siluroids, are not without traces of a similar structure. [41] Vide Steindachner, _Polypterus Lapradei_, &c., and Hyrtl, "Ueber d. Blutgefässe, &c." _Sitz. Wiener Akad._, Vol. LX. CHAPTER VII. AMPHIBIA[42]. [42] The following classification of the Amphibia is employed in the present chapter: I. Anura. {AGLOSSA. {PHANEROGLOSSA.