198 B. NATURAL HISTORY. 
mosal, not only on this account, but because, as in other Reptilia, 
it is articulated with the summit of the quadrate. 
Turning again to the posterior face of the cranium, we may be 
in a position to determine the two bones described above as lying 
outside of the occipitals, and between them and the quadrate and 
the squamosals. The superior (Op. O, fig. 2) occupies the position 
of the “external occipital” of Cuvier, in the tortoise, both by its 
articulation with the exoccipital (Ex. O) and its direction towards 
the squamosal (Sq). Its separation from the supraoccipital, and 
contact with the basioccipital, are against this determination, yet 
Fig. 1.* 
the weight of these arguments is much less than that of those for 
it; and therefore I suppose it to represent that bone, which is the 
opisthotic of modern nomenclature. 
The large foramen below the last, and exterior to the basi- 
occipital, is in the position of the opening of the internal ear in 
the Lacertilia, as regards its relation to the latter bone, the opis- 
thotic being separated from it by the extension outwards of the 
exoccipital. Its relation to the opisthofic is the same as that in the 
Cheloniidee, where it is separated from the basioccipital by an in- 
ferior process of the exoccipital. It is probably the fenestra ovale; 
and, if so, the second bone in question (stap) becomes the stapes. 
It is a question, however, to what extent this element is really 
* Fig. 1.—Ichthyosaurus ; lateral view (from specimen from Barrow, Leices- 
tershire). 
Pmx. . Premaxillary bone. Qj. ..- Quadratojugal. 
Mx... Maxillary. Q..... Quadrate. 
N. .,.. Nasal. Pob. . . Postorbital. 
Fr... . Frontal. Sq... . Squamosal. 
Prf. .. Prefrontal. _ D.....Dentary. 
Pof. . . Postfrontal. An.... Angular. 
Pa. .. Parietal. Ar. ... Articular. 
L. ... Lachrymal. 8. Ar. . Subarticular. 
M.... Malar. Pter. . . Pterygoid. 
