1885.] Progressive and Retrogressive. 239 
among the Physoclysti, characterize them as superior to the 
Physostomi, but their departure from the SEP line of the 
Vertebrata has another appearance. 
The descent of the Physoclystous fishes has probably been 
from Holostean ancestors, both with and without the intervention 
of Physostomous forms. This is indicated by increase in the 
number of actinosts in the fins of families which have pectoral 
ventral fins, as in the extinct genus Dorypterus} 
The Physostomi display three or four distinct lines of descent. 
The simplest type is represented by the order Isospondyli, and 
palzontology indicates clearly that this order is also the oldest, 
as it dates from the Trias at least. In one line the anterior dor- 
sal vertebrae have become complicated, and from an interlocking 
mass which is intimately connected with the sense of hearing. 
This series commences with the Characinide, passes through the 
Cyprinidz, and ends with the Siluride. The arrangements for 
audition constitute a superadded complication, and to these are 
added in the Siluroids defensive spines and armor. Some of this 
order, however, are distinctly degenerate, as the soft purblind 
Ageniosus, and the parasitic Stegophilus and Vandellia which are 
nearly blind, without weapons, and with greatly reduced fins. 
The next line (the Haplomi, pike, etc.) loses the przecoracoid 
arch and has the parietal bones separated, both characters of the 
Physoclysti. This group was apparently abundant during the 
Cretaceous period, and it may have given origin to many of the 
Physoclysti. 
Another line also loses the precoracoid, but in other respects 
diverges totally from the Physoclysti and all other Physostomi. 
This is the line of the eels. They next lose the connection be- 
tween the scapular arch and the skull, which is followed by the 
loss of the pectoral fin. The ventral fin went sooner. The pala- 
tine bones and teeth disappear, and the suspensor of the lower 
jaw grows longer and loses its symplectic element. The opercu- 
lar bones grow smaller, and some of them disappear. The 
_ ossification of most of the hyoid elements disappears, and some 
of their cartilaginous 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 Eurypharyngide. Here all the degenerate features above 
1 See Proceeds Amer. Assoc Adv. Science, 1878, p. 297. 
