342 THE AMERICAN NATURALIST. (VOL XXXII. 
The basis for the establishment of the two new suborders is 
found in the condition of the vertebral axis. In both groups 
the notochord may or may not be persistent. In the Proto- 
spondyli, if the notochord is “more or less replaced by 
vertebrz, the pleurocentra and hypocentra in part of the caudal 
region remain distinct even when fully developed.” In the 
Aetheospondyli, on the other hand, we find the “ pleurocentra 
and hypocentra usually fused, never forming alternating disks 
or rings.” In fact, in all the genera admitted to the latter 
suborder this fusion of vertebral elements has occurred, while 
all genera devoid of what are regarded as pleurocentra and 
hypocentra, or, possessing them, do not have them fused, are 
relegated to the Protospondyli. That is, there are no Aetheo- 
spondyli with distinct pleurocentra and hypocentra. Further- 
more, two genera which technically belong to the Aetheospondyli 
are referred justly, no doubt, to the Protospondyli. These are 
Histionotus and Neorhombolepis. We have no evidence what- 
ever presented that they possess, even in the tail, distinct and 
alternating pleurocentra and hypocentra. 
_ A word may be permitted at this point regarding relation of 
the pleurocentra and hypocentra to the notochordal sheath. 
Mr. Woodward constantly speaks of these elements as being 
developed in the sheath of the notochord.! This, so far as the 
Teleostomi are concerned, is an error, as has been demonstrated 
by the observations of many investigators. In the elasmo- 
branchs the vertebral centra are formed principally from 
cartilage which has invaded the inner sheath of the notochord, 
and this cartilage may undergo extensive calcification. The 
same appears to be true of the vertebral axis of the Dipnoi. In 
the Teleostomi no cartilage develops within the notochordal 
sheath; neither do the ossifications of the vertebral centra 
originate there. If the sheath ever becomes ossified it is a 
secondary process. 
_ Itis evident that Mr. Woodward regards as a pleurocentrum 
any ossification in or in contact with the dorsal portion of the 
notochordal sheath. He would probably further limit the defi- 
nition to ossifications arising distinct from the bases of the 
1 See pp. 80, 132, 164, 190, 195, 287, 374. 
