All known fossil Dipnoi had scales of a similar character, although differing greatly in size in different genera. In some (e.g. _Dipterus_) a layer of enamel-like substance invests the exposed portions of the scales. {193}CHAPTER VIII THE SKELETON All Fishes possess an internal skeleton which, in order that it may be distinguished from the more superficial scaly exoskeleton described in Chapter VII., is termed the endoskeleton. The latter consists (i.) of an axial part, including the vertebral column and the skull; and (ii.) of an appendicular portion, consisting of the skeleton of the limbs and their supporting pectoral and pelvic girdles. THE VERTEBRAL COLUMN.[174]—The individual segments or vertebrae which, arranged in a linear series, collectively form the vertebral column, are highly complex structures, each being formed by a number of vertebral elements, the sum total of which constitutes a vertebra. Perhaps the best conception of the nature of vertebral elements is to be gleaned from the study of such primitive Fishes as the Elasmobranchs, in which not only are all the vertebral components present, but they are less modified by suppression and fusion than in most other Fishes, and on this account they afford a convenient introduction to the study of the puzzling eccentricities of vertebral structure in other groups. Selecting any common Dog-Fish, such as _Scyllium canicula_, and starting with an early embryonic stage, it may be stated that the first indication of a vertebral column is the formation of the notochord, which, invested by its chordal sheath, extends from the tip of the tail to a point on the under surface of the brain just behind the hypophysis or pituitary body. {194}[Illustration: FIG. 111.—A, side view of precaudal vertebrae of _Scyllium canicula_; B, similar view of caudal vertebrae. _b.d_, Basi-dorsal; _c_, centrum; _h_, basi-ventral; _h.s_, haemal spine; _i.d_, inter-dorsal; _p_, parapophysis; _r_, rib; _s.d_, supra-dorsals. The vertical dotted lines indicate the limits of neuromeres and myotomes. The small circles represent the exits of the dorsal and ventral roots of spinal nerves. (After Ridewood.)]