498 



MANUAL OF ZOOLOGY. 



operculum. A pair of tubes proceed from the pharnyx to open 

 on the upper surface of the head by two apertures, which are 

 termed " spiracles," and which are sometimes regarded as the 

 homologues of the Eustachian tube and external meatus audi- 

 torius (Wyman). By means of these water can be admitted 

 to the pharnyx, and thence to the gills. 



By Professor Owen the Plagiostomi are divided into three sections, 

 termed respectively the Cestraphori, the Selachii, and the Batides. 



a. Cestraphori. In this division there is a strong spine in front of each 

 dorsal fin, and the back teeth are obtuse (fig. 270). The only living repre- 



Fig. 270. Upper jaw of Port Jackson Shark (Cestracion), showing the pavement of 

 crushing teeth. One-half the natural size. (After Owen.) 



sentatives of this group are the Port Jackson Shark (Ceslracion Philippi), 

 and some allied forms, characterised by their pavement of plate-like crush- 

 ing teeth, adapted for comminuting small Molluscs and Crustaceans. They 

 are exclusively inhabitants of the Pacific Ocean, and are remarkable for their 

 close resemblance to a large group of extinct forms, of which the best known 

 are the genera Hybodus and Acrodus from the Secondary rocks. 



b. SelachiL This group comprises the formidable Sharks and Dog- 

 fishes, and is characterised by the lateral position of the branchiae on the 

 side of the neck, and by the fact that the pectoral fins have their ordinary 

 form and position, and their anterior ends are not connected with the skull 

 by cartilages. The skull also has a median facet for the first vertebra. 



