EVOLUTION OF THE VERTEBRATA. 32T 



with the generally superior energy and tone which prevail among 

 the Physoclysti, ciiaracterize them as superior to the Physostomi, 

 but their departure from the ascending 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 basilar bones in the fins of families which have pec- 

 toral 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 

 paleontology 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 form an interlocking 

 mass which is intimately connected with the sense of hearing. 

 This series commences with the Characinid^e, passes through the 

 Cyprinidce, and ends with the Siluridae. 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 pur- 

 blind Ageniosus, and the parasitic Stegophilus and Vandellia, 

 which are nearly blind, without weapons, and with greatly re- 

 duced fins. 



The next line (the Haplomi, pike, etc.) loses the precoracoid 

 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 disappeared sooner. The 

 palatine 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. Tlie ossi- 

 fication 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 speciali- 



* See " Proceeds. Amcr. A830C. Adv. Science," 1 878, p. 297. 



