70 THE VERTEBRATE SKELETON. 



Suborder (1). SiRENOiDEi 1 . 



The head has well developed membrane bones. The trunk 

 is covered with overlapping scales and bears no bony plates. 

 Three pairs of teeth are present, two in the upper and one 

 in the lower jaw, the two principal pairs of teeth are borne on 

 the palato-pterygoids and splenials, while the third pair are 

 found in the vomerine region. The tail is diphycercal in 

 living forms. In the extinct Dipteridae it is heterocercal. 

 The pectoral girdle includes both membrane and cartilage 

 bones. The pelvic girdle consists of a single bilaterally 

 symmetrical piece of cartilage. 



This suborder is represented by the living genera Ceratodus, 

 Protopterus and Lepidosiren, and among extinct forms by the 

 Dipteridae and others. 



Suborder (2). ARTHRODIRA. 



Bony plates are developed not only on the head but also 

 on the anterior part of the trunk, where they consist of a 

 dorsal, a ventral, and a pair of lateral plates which articulate 

 with the cranial shield. The posterior part of the trunk is 

 naked. The tail is diphycercal. The jaws are shear-like, and 

 their margins are usually provided with pointed teeth whose 

 bases fuse with the tissue of the jaw and constitute dental 

 plates. There seem to have been three pairs of these plates, 

 arranged as in the Sirenoidei, the principal ones in the upper 

 jaw being borne on the palato-pterygoids. Small pelvic fins 

 are present, but pectoral fins are unknown. 



The Arthrodira occur chiefly in beds of Devonian and 

 Carboniferous age. Two of the best known genera are 

 Coccosteus from the European Devonian and Dinichthyx, a 

 large predatory form from the lower Carboniferous of Ohio. 



1 A. Giinther, Phil. Trans, vol. 161, Part 11. 1871, p. 511. T. H. 

 Huxley, "On Ceratodus and the classification of fishes," P.Z.S. 1876, 

 p. 24. 



