396 



MANUAL OF ZOOLOGY. 



under surface of the head (fig. 149, A), and there are severa. 

 branchial apertures on each side of the neck. 



Sub-order A. Holocephali. — This sub-order includes cer- 

 tain curious fishes, of which the only living forms are the 

 ChimiBridce. The notochord is persistent ; but the neural 

 arches and transverse processes are cartilaginous. The jaws 

 are bony, and are covered by broad plates representing the 

 teeth. The exoskeleton consists of placoid granules. The 

 first ray of the anterior dorsal fin is in the form of a powerful 

 defensive spine, like the " ichthyodorulites " of many fossil 

 fishes. The ventral fins are abdominal, and the tail is hetero- 

 cercal. There is only a single external gill-aperture, covered 

 with a gill-cover and branchiostegal membrane; but only a 

 small portion of the borders of the branchial laminse is free. 

 The mouth is placed at the extremity of the head. 



The best-known living representative of the sub-order is the . 

 Chimmra monstrosa (fig. 150, B), commonly known as the 



Fig. 150.— Plagioslomi and Holocephali. A, White Shark (Carcharia£\ ; 

 B, CkimtEra monstrosa. (After Gosse.) 



" king of the Herrings." In Ckimcsra there is only one ap- 

 parent gill-slit, but the gills really adhere to the integument by 



