282 SYSTEMATIC ZOOLOGY 



The general appearance of the body is sharklike except the 

 head, while the skin is smooth, without scales. There are the 

 typical shark fins well developed, with heterocercal tail, and 

 claspers on the pelvic fins in the male. The head, however, 

 is rather large, sometimes blunt, sometimes long and pointed, 

 with a small mouth near the anterior end of the head, and 

 only six teeth, four above and two below. There are only four 

 gill slits, and, unlike the other Elasmobranchii, there is a fold 

 of the skin, an operculum, which covers them so as to leave a 

 single external opening on each side of the body. There is 

 further no spiracle, and no cloaca ; the anus opens on the sur- 

 face of the body, and posterior to it is the opening of the uro- 

 genital ducts. The intestine is provided with a spiral valve as 

 in the Selachii. The eggs are laid in horny capsules which are 

 very large, in the northern genus about sixteen centimeters long, 

 in the southern, twenty-five long by seven broad. 



SUBCLASS II. TELEOSTOMI 



The Teleostomi (Gr. reXeos, complete, and arofia, mouth) 

 include the great majority of fishes, both fresh-water and 

 marine. They may be distinguished from the preceding groups 

 of Vertebrata by the skeleton, which always consists wholly or 

 partially of bone. The bones are formed in two ways ; some 

 replace the cartilaginous skeleton, while others are formed in 

 fibrous connective tissue and are called membrane bones, or 

 dermal bones, since they often develop in the deeper portion 

 of the integument. The cranium is always more or less ossified, 

 consisting of both cartilaginous and membrane bones ; and bones 

 develop in both the upper and lower jaws as a rule. The verte- 

 bras are generally ossified, and are provided with dorsal out- 

 growths inclosing the spinal nerve cord and forming the neural 

 arch, and with a pair of ventral processes, the ribs, forming the 

 haemal arch, and inclosing the viscera. 



The surface of the body ai the Teleostomi is sometimes 

 covered with a naked skin, but more frequently with scales of 

 various types, but never with placoid scales as in the Elasmo- 

 branchii. The unpaired fins are the dorsal, caudal, and ventral 

 or anal ; the dorsal and ventral vary much in size and number; 





