(71) M. Lereboullet. "Recherches d'Embryologie comparée sur le développement de la Truite." _An. Sci. Nat._, quatrième série, Vol. XVI. 1861. (72) T. Oellacher. "Beiträge zur Entwicklungsgeschichte der Knochenfische nach Beobachtungen am Bachforellenei." _Zeit. f. wiss. Zool._, Vol. XXII., 1872, and Vol. XXIII., 1873. (72*) H. Rathke. _Abh. z. Bildung u. Entwick. d. Menschen u. Thiere._ Leipzig, 1832-3. Part II. Blennius. (73) Reineck. "Ueber die Schichtung des Forellenkeims." _Archiv f. mikr. Anat._, Bd. V. 1869. (74) S. Stricker. "Untersuchungen über die Entwicklung der Bachforelle." _Sitzungsberichte der Wiener k. Akad. d. Wiss._, 1865. Vol. LI. Abth. 2. (75) Carl Vogt. "Embryologie des Salmones." _Histoire Naturelle des Poissons de l'Europe Centrale._ L. Agassiz. 1842. (76) C. Weil. "Beiträge zur Kenntniss der Knochenfische." _Sitzungsber. der Wiener kais. Akad. der Wiss._, Bd. LXVI. 1872. CHAPTER V. CYCLOSTOMATA[23]. [23] The following classification of the Cyclostomata is employed in the present chapter: I. Hyperoartia ex. Petromyzon. II. Hyperotreta ex. Myxine, Bdellostoma. Petromyzon is the only type of this degenerated but primitive group of Fishes the development of which has been as yet studied[24]. [24] The present chapter is in the main founded upon observations which I was able to make in the spring of 1880 upon the development of Petromyzon Planeri. Mr Scott very kindly looked over my proof-sheets and made a number of valuable suggestions, and also sent me an early copy of his preliminary note (No. 87), which I have been able to make use of in correcting my proof-sheets. The development does not however throw any light on the relationships of the group. The similarity of the mouth and other parts of Petromyzon to those of the Tadpole probably indicates that there existed a common ancestral form for the Cyclostomata and Amphibia. Embryology does not however add anything to the anatomical evidence on this subject. The fact of the segmentation being complete was at one time supposed to indicate an affinity between the two groups; but the discovery that the segmentation is also complete in the Ganoids deprives this feature in the development of any special weight. In the formation of the layers and in most other developmental characters there is nothing to imply a special relationship with the Amphibia, and in the mode of formation of the nervous system Petromyzon exhibits a peculiar modification, otherwise only known to occur in Teleostei and Lepidosteus.