Fossil Fislies. 



composed of distinct centra. The gills are pouch -like, and there 

 are five (six or seven) distinct clefts on each side, which are exposed, 

 having no " gill-cover," or operculum. The body is provided with 

 median and paired fins, the hinder pair being abdominal. 



In the majority of the Plagiostomes, the extremity of the ver- 

 tebral column is slightly turned upwards, and the lower lobe of 

 the caudal fin is much larger than the upper, producing a "hetero- 

 cercal " tail. In some, however, like Squatina and in several of 

 the Rays, the terminal portion of the body is straight, and the fin 

 equally developed above and below, upon the " diphy cereal '' or 

 " proto-cercal " plan. 



The skin is usually covered more or less closely by numerous 

 small detached plates or granules of dentine, with tubercles or spines 

 (Fig. 8) scattered over the whole surface of the integument, 

 commonly known as " placoid scales." When very small and close- 

 set, as in the Dog-fish, this dermal covering is called " shagreen." 



ci $ 



Fig. 8.— Dermal tubercles of Plagiostomous Fishes. 

 a, shagreen of Dog-fish [Ginglymostoma), enlarged; b, shagreen of Blue Shark 

 {Varch arias), enlarged; c, shagreen of Spiny Dog-fish (Cetttrophorus), enlarged; 

 d, dermal tubercles uf Spiny Shark [Echinorhimis), nat. size ; e, tubercle of Kay ; 

 /, dermal tubercle of Greenland Shark [Lcemaryus borealis), enlarged; g, 

 shagreen of Sting-ray ( Uroyymnus), nat. size. 



Those of the Plagiostomes with lateral gill-clefts are commonly 

 known as "Sharks" (Pig. 9), while those with depressed body 



Fig. 9. — Port-Jackson Shark, Cestracion Philippi, Lacep., Australia. 

 sp' anterior dorsal spine ; sp" posterior dorsal spine. 



and ventrally placed gill-clefts fall under the denomination of 

 " Rays " (Fig. 10). There are many intermediate forms, however, 

 which it is impossible to distinguish in a fossil state. 



