214 B. NATURAL HISTORY. 
forms of life are the more generalized in structure than the later, - 
and that this generalization is increasingly evident the further 
back we carry our inquiries. 
4. On the Homologies of the Opisthotie Bone. 
This element, distinguished by Huxley from those which compose 
with it the “temporal bone” of anthropotomists, has been called 
“mastoid” by Owen, and “ external occipital” and “mastoid” by 
Cuvier. . 
Its position is exterior to the exoccipital, posterior to the prodtic, 
and beneath and behind the squamosal. 
Fig. 9. — Chelydra serpentina ; cranium, with squamosal and postorbital bones 
removed. Epo, Epiotic; Pro, or Po, Prodtic. Ma, Meatus Auditorius. Ecp> 
Ectopterygoid. V, Foramen ovale. 
Fig. 10.— Chelone midas ; cranium from behind. (Lettering as in fig. 2.) 
