560 CARBONIFEROUS LABYRINTHODONT 
about half an inch on each side, and the lateral contours continue to 
diverge from hence to a point about 24 inches distant from the hinder 
extremity of the temporal prolongations of the skull. The external 
contour of the temporal prolongation now becomes rounded off, and 
sweeps evenly inwards, until it meets the internal contour, which 
appears to be nearly straight. The epiotic processes are not visible. 
In the middle line of the base of the skull, extending as far back- 
ward as the level of the posterior part of the supratemporal foramen, 
is the well-ossified basisphenoid, 1°7 inch broad, and slightly excavated 
posteriorly. The basisphenoid narrows anteriorly, so that, at 1°4 inch 
from its posterior extremity, it is not more than og inch wide ; beyond 
this point it suddenly widens to form the lateral processes, like those 
commonly exhibited by the basisphenoid of fishes, and then rapidly 
tapers forward, having, at 2°8 inches from its hinder extremity, a 
diameter of not more than 0:25 inch, and continuing straight and 
style-like as far as it can be traced, which is to a distance of about 
6 inches from its hinder extremity. 
The limits of the vomers cannot be accurately defined ; but they 
are very broad plates, separating the large anterior palatine foramina 
(a) from the comparatively small posterior nares (4), which are round 
apertures, I inch in diameter, and 5 inches from the anterior end of 
the snout, situated between the vomer, the maxilla, and the palatine 
bones on each side. The vomers unite in the middle line with one 
another and with the prolonged anterior extremity of the basisphenoid. 
Posteriorly they are connected with the palato-pterygoid arcade, the 
separate components of which cannot be accurately defined. The 
palatine portion, however, is a broad, flat plate, measuring 3°5 inches 
between the posterior nares and the palato-temporal foramen. It is 
united externally with the maxilla. Internally it is separated by a 
narrow interval from the basisphenoid. Posteriorly it passes into the 
pterygoid portion, which is narrow and curves outward, beneath the 
inner contour of the temporal prolongation of the skull, to be lost 
about the posterior and internal angle of that region. Externally the 
margin of the pterygoid portion is arcuated, to form the boundary of 
the palato-temporal foramen. Through this foramen the under surface 
of the upper wall of the skull in the temporal region becomes visible. 
The sutures separating the component bones of this region are not 
visible ; but on the level of the posterior end of the basisphenoid it 
presents an elongated aperture, or supratemporal foramen (c), 1°3 inch 
long by o4 inch wide. The long axis of this foramen is directed 
obliquely forwards and inwards, and it is nearer the pterygoid than 
the external boundary of the palato-temporal foramen. 
