118 



PALAEONTOLOGY 



Order III.— GANOIDEI. 



Char. — Endo-skeleton in some osseous, in some cartilaginous, 

 in some partly osseous and partly cartilaginous ; exo- 

 skeleton formed by enamelled bones ; fins usually with 

 the first ray a strong spine. 



Sub- Order 1 .— PLACOGANOIDEI. 



(Ostracostei, 0\v. Cat. Mus. Coll. of Surgeons, 4to, 1854, p. 166.) 



Char. — Endo-skeleton cartilaginous, or retaining the noto- 

 chord ; head and more or less of the trunk protected by 

 large ganoid, often reticulated, plates ; heterocercal. 



The last term signifies a form and structure of tail illus- 

 trated by fig. 42, and to be seen in the sharks, dog-fishes, and 

 sturgeons of the present day : it results from a prolongation 



3£i^ 



dh 



Fig. 4'2. 

 Heterocercal tail (Lepidosteux osseus). 



■ it the vertebral column into the upper lobe dn, producing an 

 unsymmetrical form of the caudal fin, which is contrasted with 

 the symmetrical form of the same fin presented by most fishes 

 of the present day, and illustrated by the Lc/>fn/<]>is xpratl-i- 

 Jormis (fig. 56, p. 144V), in which the vertebral column termi- 

 nates at the middle of the base of the caudal fin. There 

 are a few exceptional intermediate forms and structures of 

 this fin. 



Fam. Placodennata. — The fossil remains of the singular 



