10 



FOSSIL FISHES. 



SUB-CLASS III. PAL.EICHTHYES. 



Order I.— Choxdeopteuygii (Sharks, Rays, Chimaeras). 



Sub-order I Plagiostomata. 



SelacJioidei — Sharks. 



Wall-cases 1, 2, 3, and Table-cases 25 to 33. — In these fishes the 

 skeleton is cartilaginous, the skull consists of a simple box, the 

 separate parts of which are not indicated by sutures; the teeth 

 are very varied in form and consist of vaso-dentine, they are fixed 

 in rows and by ligament to the cartilaginous jaws. The backbone 

 is sometimes composed of distinct vertebrae, but is frequently 

 cartilaginous or notochordal. The gills are pouch-like and open on 

 the surface as a series of separate clefts or apertures, placed on the 

 sides of the neck in the Sharks, and beneath the bodv in the Ravs. 

 The gill-openings have no external gill-cover or operculum, nor are 

 there any branchiostegal rays. The body is provided with median 

 and paired fins, the hinder pair being abdominal. 



The skin is usually covered, more or less closely, by numerous 

 small detached plates or granules of dentine, with tubercles or 

 spines (Fig. 22) scattered over the whole surface of the integument; 



FORMS OF TAILS OF FISHES. 



Fig. 7.— Homocorcal. * 

 Modern form. 



Fig. 6.— Heterocercal. t 

 Ancient form. 



Fig. 5.— Diphycercal. J 

 Primitive form. 



when very small and close-set, as in the Dog-fish and the Shark, 

 this dermal covering is called "shagreen." The denticles agree 

 closely in microscopic structure with true teeth. 



In the majority of the Chondropterygians the extremity of the 

 vertebral column is bent upwards and continued along the upper 

 lobe of the tail-fin, which is usually considerably prolonged, hence 

 called " heterocercal " (Fig. 6) ; only a few, like Sqimtina and some 

 Rays and Chimeras, possess a " diphycercal" tail, in which the fin is 

 equally developed on both sides of the vertebrae, forming a simple 

 pointed caudal fin (Fig. 5), as in the " Mud-fish," Protopterus. 



In most modern fishes, such as the Mackerel, Herring, and 

 Salmon, the tail is " homocercal," the fin-rays being equal-lobed 

 both above and below the vertebrae (Fig. 7). 

 ^ Of these three forms there can be no doubt that the Diphycercal 



h Homocercal, equal-lobed tail-fin. f Heterocercal, unequaldobed tail-fin. 



X Dvphycercal, double tail-fin. 



