328 THE STEUCTUEAL EVIDENCE OF EVOLUTION. 



zation and degeneracy is seen in the abyssal eels of the family 

 Eurypharyngidas. Here all the degenerate features above men- 

 tioned are present in excess, and others are added, as tlie 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 Physoclysti nearest the Physostomi have abdominal ven- 

 tral fins, and belong to several orders. It is such types as these 

 that may be supposed to have been derived directly from Holos- 

 tean ancestors. They appear in the Cretaceous period (Derceti- 

 dae), along with the types that connect with the Physostomi 

 (Haplomi). Intermediate forms between these and typical Phy- 

 soclysti occur in the Eocene (Trichophanes, Erismatopterus), 

 showing several lines of descent. The Dercetidas belong appar- 

 ently 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 Hemi- 

 branchi is a series of much interest. Its members lose the mem- 

 brane of their dorsal spinous fin (Gasterosteidae), and then the fin 

 itself (Fistularia, Pegasus). The branchial apparatus has under- 

 gone, as in the eels, successive deossification (by retardation), 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 more downward step is seen in the next 

 succeeding order of the Lophobranchii. The branchial hyoid 

 apparatus is reduced to a few cartilaginous pieces, and the bran- 

 chial fringes are much reduced in size. In the Hippocampidae 

 the caudal fin disappears and the tail becomes a prehensile organ 

 by the aid of which the species lead a sedentary life. Tlie mouth 

 is much contracted and becomes the anterior orifice of a suctorial 

 tube. This is a second line of unmistakable degeneracy among 

 true fishes. 



The Physoclysti with pectoral ventral fins present us with per- 

 haps ten important ordinal or subordinal divisions. Until the 

 paleontology of this series is better known, we shall have diffi- 

 culty in constructing phylogenies. Some of the lines may, how- 

 ever, be made out. The accompanying diagram will assist in un- 

 derstanding them. 



The Anacanthini present a general weakening of the organiza- 

 tion in the less firmness of the osseous tissue and the frequent 

 reduction in the size and character of the fins. The caudal vor- 



