152 



CHAPTER XXXIL 



ORDERS OF FISHES {Continuecl). 



Order III. Elasmobeanchii ( = SelacMa, Miiller ; Placoidei, 

 Agassiz ; Holoccpliali and Plagiostomi, Owen). — This order 

 includes the Sharks, Eays, and Chimserte, and corresponds 

 with the greater and most typical portion of the Chondrop- 

 tcrygidoi or Cartilaginous fishes of Cuvier. The order is 

 distinguished by the following characters : The shidl and 

 lower jaw are ivell developed, hut there are no cranial hones, and 

 the skvll consists of a simple cartilaginous hox, ivithout any in- 

 dication of sutures. The vertebral column is sometimes com- 

 'posed of distinct vertehrce, sometimes cartilaginous or stih-noto- 

 chordal. The exoskeleton is in the form of placoid granules, 

 tid)ercles, or spines. There are two pairs of fins, representing 

 the limhs, and supported hy cartilaginous fin-rays ; and. the 

 ventral fins are placed far hack near the anus. Tlie pectoral 

 arch has no clavicle. The heart consists of a single atiriele and 

 ventricle, and the hidhus arteriosus is rhythmically contractile, 

 is provided tvith a special coat of striated muscidar fibres, and 

 is furnished ivith several transverse rows of valves. The gills 

 are pouch-like. 



In most of the above characters it will be seen at once 

 that the Ulasmohranehii agree with the Ganoid fishes, espe- 

 cially as regards the structure of the heart. The following 

 points of difference, however, require more special notice : — 



1. The coMskcleton is what is called by Agassiz "placoid." 

 It consists, namely, of no continuous covering of scales or 

 ganoid plates, but of more or less numerous detached grains, 



