54 



^OOLOGY. 



Ganoid scales consist of a layer of bone covered with 



a coating of enamel. They are thicker and larger than 



the other kinds of scales. They are generally rectan- 



gular in form, and instead of 



overlapping, are in contact at 



the edges, like tiles in a floor. 



Few living fishes have ganoid 



scales, but they are character- 



istic of fossil fish, especially 



those found in the primary 



rocks. The sturgeon is a 



modern example. 



4. Placoid (Gr. plax, a 

 plate ; eidos, form). These 

 scales consist of bony grains 

 or plates scattered over the 

 surface of the skin. The plates 

 are sometimes furnished with 



ipiifp ctninpQ Thpv arp ffvnnrl 

 acuie spines. J.ney a e lounu 



111 the sharks and. rays. 



67. Development. Fish are generally oviparous ani- 

 mals. The ovaries, which are known by the name of roe> 

 are of large size, and sometimes contain an immense 

 number of ova. The male organs are called "milt" or 

 soft roe. The ova, after deposition in the water, are 

 impregnated by the milt. 



Some fish are ovoviviparous. 



FIG. 20. FISH SCALES. 



c cycloid scale of the carp; a, 

 ctenoid scale o the perch . d 



ganoid scales; e, placoid scale. 



QUESTIONS. II. 



1. What is the derivation of Mammalia ? 



2. What are the most important characters of this class ? 

 -3. What are the principal regions of the vertebral column ? 



4. What is the general number of the cervical vertebra, and 



what are the exceptions ? 



5. How many dorsal vertebrae are there ? 



6. How is the cavity of the chest formed ? 



7. What is the sternum ? 



8. Where are the lumbar and sacral vertebrse situated 

 .9. What is meant by " anchylosed " ? 



