112 CHIMAEROIDS 
beak-like jaws of the Devonian Rhynchodus (Fig. 111), 
of the Devonian Ptyctodus, or of the Mesozoic genera, 
e.g. Ischyodus (Fig. 112), differ little in their structures 
from those of their living kindred (Figs. 109, 109 A, 110). 
The tritors accordingly are only doubtfully to be derived 
from the fusion of the primitive basal substance of the teeth 
with the tissue of the jaws. But the history of Chime- 
roids tells of their ancient importance and of the diversity 
of their forms, and demonstrates that they cannot be con- 
nected with other existing forms of fishes. In Liassic 
times their specialized members bore the same relation to 
Chimera as did the aberrant Cestracionts of the Coal 
Measures to the simpler sharks. In their dental evolution 
they had even reached a more specialized condition than 
the Cochliodonts (Cestracionts?). Thus in Myriacanthus 
and Squaloraja, ‘all anterior prehensile teeth have disap- 
peared, and the growth of the dental plates, instead of 
taking place exclusively at the inner border, seems to have 
gradually extended to the whole of the attached surface. 
The Chimeeridz exhibit an advance in the circumstance 
that all the dental plates are thickened, while the hinder 
upper pair are both closely apposed in the median line and _ 
much extended backward” (Smith Woodward).* Squaloraja 
had certainly attained a high degree of evolution in the 
calcified vertebral rings, and in its specialized girdles, fins, 
and clasping organs. Myriacanthus, on the other hand, 
while retaining its ancient vertebral characters, had evolved — 
a well-marked series of membrane bones. . 
One cannot deny that the study of Chimzroids as a 
group emphasizes many of their structural affinities to 
the sharks. They resemble them in their cartilaginous 
skeleton, fins and girdles, “claspers,” integument, and 
* Cat. Fossil Fishes Il, xvi. 
