72 HIGH SCHOOL ZOOLOGY. 



Fig. 46. — ^The Porcupine Fish. Chilomycteni^ geometricus, J. 

 (U. S. F. C.) 



24. By fai' tlie greater number of the Fishes of the present 

 day belong to the Sub-Class Teleostei, but in past geological 

 times such was not the case, and large numbers of fossil forms 

 are known which indicate that the other sub-classes, which aie 

 but sparingly represented by living forms, were at one time as 

 abundant as the Teleosts are now. One of these Sub-Classes, 

 the Ganoidei, we have exceptional opportunities for studying on 

 this Continent, because out of the nine genera six ai'e American. 

 The peculiarities which distinguish the Ganoids from the 

 Teleosts may be best leai'ned by comparing any one of them 

 with a catfish, which of all the Physostomi comes nearest to the 

 Ganoids. As to its skin the Ganoid is rarely smooth, but 

 generally covered with bony plates or scales which may be rough 

 with teeth or smooth with enamel ; the skeleton is cartilaginous 

 in the Sturgeon, but as well ossified in the Garpike and Amia 

 as it is in the Catfish. The heart has a muscular arterial cone 

 with several rows of valves ; the pseudobranch of the Teleosts 

 may be either present as such, or, as in the Sturgeon, as a 

 functional half-gill on the hyoid arch. A gill-slit without any 

 functional gill persists in the form of the "spiracle" in the 

 Sturgeon between the hyoid and the mandibular arch, and is 

 more or less complete in the other forms, but the other gill-slits 

 are concealed as in the Teleosts by a gill-cover. The air-bladder 

 opens by a wide duct into the cesophagus and is very richly 

 supplied with blood, so that in some forms it acts as an accessoiy 



