592 SYSTEMATIC RELATIONS OF FISHES. 
These orders will be more fully defined, and the families which 
are referable to them pointed out. 
GENERAL OBSERVATIONS. 
In tracing the affinities of the Physostomi, I have pointed out . 
the relation between the Chrondrostei and the Nematognathi, and 
between the Halecomorphi and the Isospondyli. The first named 
of each of these pairs are the structural, and probably genetic, 
predecessors of the second. The series commenced with the cat 
fishes may be continued into the Mormyri and then to-the families 
of the Plectospondyli, where the series with altered vertebrz and 
with ossicula auditus terminates. The Characins have, however, 
considerable affinity to the Isospondyli, especially in the type of 
their branchial bones. From the latter group we pass to the Ha- 
plomi, and thence to the Physoclyst groups. The eel-like groups 
form a special line. The Glanencheli have cranial characters of 
the groups with modified vertebra, with fins of the more typical 
eels. The latter show a steady approach in some points to the 
conditions characterizing the Chondrostei. The loss of the maxil- 
lary, of opercular bones, and of pharyngeal elements, reminds one 
of these, but in the loss of the premaxillary, and great develop- 
ment of the ethmoid, in the Colocephali, we have features quite 
unique. The vertebral position of the scapular arch is the only 
shark character they possess; while on the other hand, the Holos- 
tomi are undoubtedly related to the Mastacembelus, a real Physo- 
clyst with spinous dorsal fin. These relations are as yet entirely 
inexplicable. 
The affinities among the Physoclysti are more clear. Omitting 
the genus just mentioned, we find the four orders with ventral fins 
to form a true series, with a Synentognath variation, terminating 
in the greatly degraded order of Lophobranchii. The Percesoces 
give us our nearest connection with the groups with abdominal 
ventral fins, and lead at once to the Percomorphi. From this cen- 
tre radiate many lines of affinity. One leads from the Chcetodon- 
tide, through the Acroneuride and to the Plectognathi, by the 
similarity in the arrangement of the posttemporal and forms of 
the pharyngeal apparatus. An important division of the Perco- 
morphi has the basis cranii simple and the branchials reduced 
above ; viz., the Scyphobranchi. The Cottide are the most gener- 
alized family of this group, and lead on the one hand to the Tri- 
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