io 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of the lower jaw grows longer and loses its symplectic 

 element. The opercular bones grow smaller, and some 

 of them disappear. The ossification of most of the 

 hyoid elements disappears, and some of their cartila- 

 ginous bases even vanish. These forms are the marine 

 eels or Colocephali. The most extraordinary example 

 of specialization and degeneracy is seen in the abyssal 

 eels of the family Eurypharyngidae. Here all the de- 

 generate features above mentioned are present in ex- 

 cess, and others are added, as the loss of ossification 

 of a part of the skull, almost total obliteration of the 

 hyoid and scapular arches, and the semi-notochordal 

 condition of the vertebral column, etc. 



The Acanthopterygia nearest the Malacopterygia 

 have abdominal ventral fins, and belong to several or- 

 ders. It is such types as these that may be supposed 

 to have been derived directly from Holostean ances- 

 tors. They appear in the Cretaceous period (Derce- 

 tidae), along with the types that connect with the Ma- 

 lacopterygia (Haplomi). Intermediate forms between 

 these and typical Acanthopterygii occur in the Eocene 

 (Trichophanes, Erismatopterus), showing several lines 

 of descent. The Dercetidae belong apparently to the 

 order Hemibranchi, while the Eocene genera named 

 belong apparently to the Aphododiridae, the immediate 

 ancestor of the highest Physoclysti, the Percomorphi. 

 The order Hemibranchi is a series of much interest. 

 Its members lose the membrane of their dorsal spinous 

 fin (Gasterosteidae), and then the fin itself (Fistularia, 

 Pegasus). The branchial apparatus has undergone, 

 as in the eels, successive deossification (by retarda- 

 tion), and this in direct relation to the degree with 

 which the body comes to be protected by bony shields, 

 reaching the greatest defect in the Amphisilidae. One 



