AFFINITIES OF CHIM/EROIDS 115 
derived from its conditions as ancestral. The dentition 
of Chimeeroids alone is so remarkable that no direct proc- 
ess of differentiation could convert it into the structures of 
lung-fish or Ganoid. A number of archaic features draw 
fishes together in the lines of their descent, but they can- 
not be interpreted as linking the Chimzroids with the 
Dipnoans, or the Dipnoans with the Chimzroids. Auto- 
stylism, often adduced to ally these groups, differs widely 
in its characters in each (p. 254): and the apparent similar- 
ities in dental plates and membrane bones are closely 
paralleled by the sharks. The diphycercal tail of the 
Chimezeroid can be made no standard of comparison, since 
it is evidently a secondary structure, arising within the 
limits of the group, as it may well have done among 
’ sharks (Pleuracanthus) or Teleostomes (Polypterus, eel). 
If the sum of the general characters of Chimzroids be 
considered, their affinities would clearly be to the most 
ancient sharks. Their structures are not so widely at vari- 
_ance with those of Elasmobranchs that they cannot rea- 
sonably be derived from their more generalized conditions 
in vertebral characters, cranium, mandible, girdles, fins, 
membrane bones, gills. Absence of swim-bladder is again 
strikingly shark-like. Like the ancient sharks, they have 
been well adapted for survival by evolving but few special- 
ized structures (e.g. dentition, gills). Their ventral clasp- 
ing organs separate them clearly from the Dipnoans. 
Until the discovery of Harriotta the frontal clasping spine 
_ remained as one of the most distinctive features of Chi- 
_ meeroids ; its high degree of specialization in Liassic times 
is alone significant of the antiquity of their descent. 
