xx] 



PHOTOS POND Y LI 



499 



dorsal with the basiventral. It is termed the postcentrum. The 

 archless centrum or precentrum is formed chiefly by a bony ring 

 connecting the dorsal intercalaries with the ventral intercalaries. 

 Where the tail merges into the trunk the precentrum may be 

 seen to become attached to the postcentrum. A vertebra of 

 the trunk consists, therefore, of the basidorsal of one myotome 



FIG. 243. Three stages in the development of the vertebral column of Amia. 



A. Stage when basidorsals, basiventrals and intercalaries are separate. 



B. Stage when basidorsals and basiventrals are connected by a ring of bone, 

 and likewise dorsal and ventral intercalaries. 



C. Stage of formation of complete vertebra in the trunk and of precentrum 

 and postcentrum in the tail. The hinder part of the figure shows the 

 condition in the tail, the front that in the trunk. 



1. Basidorsal. 2. Notochord. 3. Basiventral. 4. Dorsal inter- 

 calary. 5. Ventral intercalary. 6. Bony ring connecting basidorsal 

 and basiventral. 7. Bony ring, connecting intercalaries. i. Arch- 

 less precentrum. n. Arch-bearing postcentrum. in. Complete 

 vertebra in the trunk. Dotting, etc. as in previous figure. 



and of the basiventral and the dorsal and ventral intercalaries 

 of the myotome in front. 



The tail fin agrees in shape with that of Lepidosteus, but fulcra 

 are absent, since the scales of Amia are "cycloid," i.e. thin and 

 devoid of both ganoin and placoid denticles. 



322 



