io8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of vertical plates like the leaves of a book. These 

 characters are further developed in the Plectognathi. 

 The brain-case is very small, the face is very elongate, 

 and the mouth is much contracted. The bones sur- 

 rounding it in each jaw are coossified. The axial ele- 

 ments (pubes) of the posterior fins unite together, be- 

 come very elongate, and lose the natatory portion. In 

 one group (Orthagoriscidae) the posterior part of the 

 vertebral column is lost, and the caudal fin is a nearly 

 useless rudiment. In the Ostraciontidae (which may 

 have had a different origin, as the pharyngeal bones 

 are not contracted) the natatory powers are much re- 

 duced, and the body is inclosed in an osseous carapace 

 so as to be capable of very little movement. The en- 

 tire order is deficient in osseous tissue, the bones be- 

 ing thin and weak. It is a marked case of degeneracy. 



There are several evident instances of sporadic de- 

 generacy in other orders. One of these is the case of 

 the family of the Icosteidae, fishes from deep waters off 

 the coast of California. Although members of the 

 Percomorphi, the skeleton in the two genera Icosteus 

 and Icichthys is unossified, and is perfectly flexible. 

 Approximations to this state of things are seen in the 

 parasitic genus Cyclopterus, and in the ribbon-fishes, 

 Trachypteridae. 



Thus nearly all the main lines of the Acanthopte- 

 rygii are degenerate ; the exceptions are those that 

 terminate in the Scombridae (mackerel), Serranidae, 

 and Scaridae (Pharyngognathi). 



c. The Line of the Batrachia. 



We know Batrachia first in the Coal Measures. 

 They reach a great development in the Permian epoch, 

 and are represented by large species in the Triassic 



