292 PRACTICAL LESSONS IN SCIENCE. 



tened from above downward, but they are usually "compressed," 

 flattened from side to side ; as in the case of the bass or perch. 



The fins are membranous organs, probably folds of skin, sup- 

 ported by spines or rays. The principal one is the caudal or tail 

 fin. Of these there are two kinds, the hoinocercal, when the back- 

 bone ends at the base of the fin and the fin is symmetrical, and the 

 heterocercal, when the backbone extends into the upper lobe of 

 the fin, making it unsymmetrical, as in the sturgeon and shark. 

 The fin along the center of the back is the dorsal; it is sometimes 

 single, sometimes double, and it may be supported by spines or 

 soft rays. Then there are the pectoral or breast fins, the ventral 

 fins and theana7fin. Count the spines in each of the fins, count the 

 soft rays, measure along the base to find the length of the fin, 

 and the length of the longest ray to get the height of the fin. 

 Compare the fins as to length, height, number of rays and spines, 

 and note their relative position. Close the mouth and measure 

 from the tip of the snout to the posterior margin of the bone 

 which covers the gill opening for the length of the head, and to 

 the base of the caudal fin for the length of the fish, and measure 

 from above downward at the widest place for the depth of the 

 fish. Compare the length of the head with the length of the body, 

 and the depth with the length and width, etc. 



The bone that serves as an upper lip for the fish is the premax- 

 illary; note its form, size, and movements; notice also the fine 

 teeth on it, and their shape, size and arrangement. Back of the 

 premaxillary on each side are the oblong maxillary bones, and 

 abovethese in the median line are the nasal, prefrontal and frontal 

 bones, and on the sides of the lower jaw are the dentary bones. 



In the front part of the roof of the mouth find a group of teeth 

 situated on a bone called the vomer. Extending backward on 

 each side of the roof of the mouth find rows of teeth located on 

 the palatine bones. Examine other parts of the mouth and the 

 tongue for teeth ; compare the different sets as to form , size and 

 arrangement. In front of the eye is the anteorbital and below it 

 are the suborbital bones. 



The cavities in the sides of the fish which divide the head from 



