83 
each end to the wall of the chamber. Water enters the 
respiratory chamber by five to seven lateral openings in the 
case of the Plagiostomi, by a single aperture in Chimcera, 
It will be convenient to contrast this with the Teleostean 
branchial apparatus. Here we have gills supported on 
branchial arches, but not attached to each other or to 
the sides of the gill-cavity. The outer wall of the 
respiratory chamber is formed by a moveable plate, the 
operculum or gill-cover. "We see that Ganoids with their 
free gills and gill-cover resemble the Teleostei and recede 
from the Elasmobranchii so far as their respiratory apparatus 
is concerned. There are, however, variations and anomalies 
which may induce us to hesitate before attaching much classi- 
ficatory weight to these resemblances and differences. Polyp- 
tcrus and CalamoicMhySf with their single rudimentary 
spiracle on each side of the head remind us strongly of 
Chimcera, and Ceratodus, besides the spiracle, has each of its 
four gills attached externally to the wall of the gill-cavity. 
Lastly, Miiller gives the abdominal position of the ventral 
fins as an easy external mark of the Ganoid order. This is 
plainly not of great importance ; it is the characteristic of 
one of Cuvier's sections of Teleostei or Osseous fishes, the 
Malacopterygii Abdominales. Moreover, since 1844, Cala- 
moichthys has been brought to light, a Ganoid with no ventral 
fins at all. Miiller seems to have contemplated such a con- 
tingency, as we see from the words — "Den Character von 
den abdominalen Bauchflossen halte ich bloss zeitweilig fiir 
bindend." 
Such are the never-failing and absolute characters of the 
Ganoids known to Miiller. Besides these, there are other 
features which require attention. 
Like Lejndosiren and the Elasmobranchii the Ganoids have 
a spiral valve in the intestine. This structure, the function 
of which is imperfectly known, may be described as an open 
