[1] The skin consists of an outer layer of epiblastic origin, the epidermis, and an inner layer of mesoblastic origin, the dermis. The epidermis is divided into two principal layers, an outer one, the horny layer or _stratum corneum_, and an inner one, the _stratum Malpighii_. The innermost part of the stratum corneum is distinguished as the _stratum lucidum_, and the outermost part of the stratum Malpighii as the _stratum granulosum_. [2] The enamel of the pharyngeal teeth of some Teleosteans is hypoblastic in origin. [3] See also p. 71. [4] It is usual to regard the clavicle as a membrane bone, but Kölliker has shown that in rabbit embryos of about the 17th day it is cartilaginous. [5] In compiling these paragraphs on Histology, free use has been made of Klein and Noble Smith's _Atlas of Histology_, the small Histologies of Klein and Schäfer, Huxley's _Elementary Physiology_, and Lloyd Morgan's _Animal Biology_. [6] See Huxley's _Elementary Physiology_, Revised edition, London, 1886, p. 180. [7] Strictly speaking the jaws, visceral skeleton, ribs and sternum do not form part of the axis, but it is convenient to group them as parts of the axial skeleton. [8] F.M. Balfour, _Comparative Embryology_, vol. II., London, 1881, p. 465. W.K. Parker and G.T. Bettany, _The Morphology of the Skull_, London, 1877. [9] Sometimes also called ectethmoids or parethmoids. [10] The _proximal_ end of anything is the one nearest the point of origin or attachment, the _distal_ end is the one furthest from the point of origin or attachment. [11] W.K. Parker, _A Monograph of the Shoulder Girdle and Sternum_, Ray Soc. London, 1868. [12] See R. Wiedersheim, _Zeitschr. wiss. Zool._ vol. LIII. suppl. p. 43, 1892. [13] G. Baur, _Beiträge zur Morphogenie des Carpus und Tarsus der Vertebraten_, Theil 1. Batrachia. Jena, 1888, and _Amer. Natural._, vol. XIX. 1885 (several papers). [14] This account is based on Chapter XX. of Flower's _Osteology of the Mammalia_. London 1876. CHAPTER II. CLASSIFICATION. THE following classification includes _only the forms mentioned in the succeeding pages_. The relative value of some of the terms employed in classification is not identical throughout the book. This remark applies specially to the term _group_, which is a convenient one, owing to its not having such a hard and fast zoological meaning as has the term _family_, for instance. The term _group_ is applied in this book to divisions of the animal kingdom of very different classificatory importance. PHYLUM CHORDATA. SUBPHYLUM A. HEMICHORDATA. Balanoglossus. Cephalodiscus. Rhabdopleura. ? Phoronis. (? Actinotrocha--larval Phoronis). SUBPHYLUM B. UROCHORDATA (TUNICATA). Group LARVACEA and others. SUBPHYLUM C. CEPHALOCHORDATA. Amphioxus--lancelet. NOTE. In this chapter all the generic names printed in italics are those of extinct animals.