OF BAL^NOPTERA MUSCULUS. 207 



springing from a comparatively narrow base. It joins the exoccipital above tbe petrous 

 bone, as already described ; its internal edge is then free, till it meets the pterygoid, where 

 it turns at a right angle and ascends, helping to form the external opening of the canal 

 leading from the foramen ovale. The posterior border consists internally of a prominent 

 ridge, which forms the posterior margin of a deep groove anterior to that for the petrous. 

 Further out this groove passes into the upper surface as a large mass projects below it. 

 The anterior border of the articular portion is a smooth vertical plate ; towards the outer 

 part of the lower edge there is a tuberosity. The external border is thick and curved. A 

 straight line between its ends measures. 27 1-2 inches, one following the superior convexity 

 41 inches. The superior surface is convex, marked towards its posterior end by several 

 prominences ; internally it is overlapped by the supra-occipital. The curve described is 

 greater than I have seen figured in the B. musculus and is more like that of the B. 

 rosfrata. 



The temjyoral i^ortion passes forward from the anterior side of the articular portion. 

 The superior edge is at first covered by the supra-occipital, but its anterior half is separated 

 from it by a thin plate of the parietal. The anterior border of this portion presents two 

 semilunar edges ; the upper joins the parietal, the lower the alisphenoid, and the common 

 angle projects between these bones. A very peculiar appearance must now be mentioned; 

 a portion of this bone is in part cut off from the rest by sutures as definite as those separat- 

 ing distinct bones. A suture begins a little below the points of union of the two semilunar 

 edges and runs upward and backward, parallel with the lower horn of the sujjerior half- 

 moon for about 6 inches, where, forming an acute angle with its previous course, it turns di- 

 rectly downward and ends suddenly in a foramen before reaching the lower border. Ex- 

 cept in some trifling details this appearance is the same on the two sides. 



The Periotlc (or Petrous) Bone (PI. vi, fig. 2 c). — As this bone has been removed fi-om 

 one side of the skull for the purpose of studjdng the internal ear, its minute description will 

 be given in that connection, and it will here be sufficient to allude to its relations with the 

 surrounding bones. 



Tlie bulla is very pi'ominent ; its long diameter is directed forward and a little outward 

 The convex aspect faces outward and downward, and the edge of the carina bounds the fo- 

 ramen lacerum poster ius. The posterior portion is seen pointing outward and backward, 

 tightly wedged in between the squamous and exoccipital. The lower part of the anterior 

 or pyramidal portion is seen as it enters a cavity between the squamous and pterygoid. A 

 little of the labyrinthic portion is also seen in the depths of the fossa. 



The AUs2)henoid (PL vi, fig. 2 rZ). — As much of this bone as is visilde from the outside 

 may be divided into three parts, viz., an inferior horizontal portion on the base of the skull, 

 an external one in the temporal fossa, and a pterygoid portion springing in part from each 

 of the others. The inferior portion is about 12 inches long, articulating internally with the 

 vomer. Anteriorly it is overlapped by the palatal, tiU at the middle of its outer border it 

 bends down to form the internal pterygoid plate ; behind, this the border is free. The 

 external portion of the bone is situated in the deepest part of the temporal fossa. It is 

 bounded behind by the squamosal, above and in front by the parietal and the small inter- 

 posed scale of the basisphenoid. The surfoce is here concave, but below, as it grows nar- 

 rower, it becomes convex and makes a bold turn downward and inward forming the ex- 

 ternal pterygoid plate, which is in contact with the palatal bone anteriorly. The base of 



