Class IV.~PISCES. 
[The True Fishes.) 
Skeleton bony or cartilaginous. Skull with sutures ; a lower jaw pres- 
ent; membrane bones develoi^ed in connection with the skull ; gill-open- 
iugs a single slit on each side, sometimes confluent ; gills attached to bony 
arches, normally four pairs of them, their outer margins free. Median 
and paired fins usually developed; a lyriform shoulder- girdle. Exo- 
skeleton various. Ova comparatively small. No “claspers”. Brain- 
differentiated and distinctly developed. Heart developed, divided into 
an auricle, ventricle, and arterial bulb. 
As here understood, this groui) includes the great majority of recent 
fishes, and is equivalent to the Teleostei, Ganoidei, and Dipnoi of most 
recent writers, the Actinopteri, Crossopterygia, and Dipnoi of Professor 
Cope. Omitting all notice of the Dipnoi and the Crossopterygia, all the 
members of which groups are extralimital and confined to the fresh waters 
of the tropics, it will be convenient to divide the American representa- 
tives of the class of Pisces into four series or subclasses — Cliondrostei, 
llolostei, Physostomi, nml Physoclisti, the first and second including most 
of the Ganoidei of Miiller, the second and third the Teleostei. These 
groups are evidently of unequal value, the Pliysostond and the Physo- 
clisti being very intimately connected, and the relations of the Holostei 
with the Physostomi are probably more intimate than their relations 
with the other Ganoids, as has been shown by Professor Cope. Never- 
theless, these groups exist in nature, and their recognition under one 
name or another is convenient. 
ANALYSIS OF THE SUBCLASSES OF PISCES. 
a. Arterial biilb muscular, with numerous valves ; optic nerves forming a chiasma ; 
ventral tins abdominal; au’-bladder with a duct ; tail beterocei’cal. 
(Series Ganoidei.*) 
h. Ventral fin with an entire series of basilar segments; skeleton cartilaginous. 
ClIONDIlOSTEI. 
hi. Ventral fin with the basilar segments rudimental; skeleton bony. . Holostei. 
ydvog, splendor ; from the enamelled scales. 
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