480 SCHULTE, SKI WHALE. 



postorbital plate with the descending process of the preorbital. This arch abuts against the 

 flat cartilaginous orbito-sphenoid, which overlaps the frontal on its cranial surface, and mesally 

 completes the roof of the orbit. The distance between the ends of the frontal arch just described 

 is 10 mm., that between the lateral extremities of its pre- and postorbital processes is 21 mm. 

 The depth of the orbital plate is 11 mm. It is thus evident that the orbital plate of the frontal 

 has a more flaring form, and in particular is transversely far shorter than in the adult. A 

 further difference lies in the complete exposure of its orbital surface in its whole extent, and the 

 absence of any overrolling on the part of the preorbital plate or of the postorbital, such as occurs 

 in the subsequent transverse lengthening of the orbital process, with the result of reducing the 

 orbital roof to a small triangle at its lateral end, and the concealment and exclusion of the orbito- 

 sphenoid from this secondary orbital cavity. 



The floor of the orbit is almost wholly membranous. Ventral to the preorbital plate of the 

 frontal a triangular surface of maxilla presents in the floor and articulates with the zygoma. 

 The latter is very slender, ventrally convex, slightly expanded at its ends, having mesal and 

 lateral margins and dorsal and ventral surfaces. To its mesal margin the suborbital aponeurosis 

 is attached. At the mesal extremity of the orbit, the external pterygoid plate forms a ledge 

 ventrally comparable to the frontal arch dorsally, but less regular and lying in a more lateral 

 position relative to the ala orbitalis and the body of the sphenoid, to which latter it is here joined 

 by a considerable mass of connective tissue. Caudad the margin is completed by the ala tem- 

 poralis and by the ventral angle of the parietal, the margin of which participates in the ledge and 

 continues it to the mesal extremity of the postorbital plate of the frontal. The ledge thus con- 

 stituted serves for the mesal attachment of the suborbital aponeurosis, which caudally joins 

 the edge of the postorbital plate and rostrad is connected to the orbital process of the maxillary, 

 the preorbital plate of the frontal, and its descending process. As a whole the floor of the 

 orbit is less convex than the roof, it is however longer transversely, for the zygoma is more 

 laterally placed than the receding supra-orbital margin. The intimate relation of the floor of 

 the orbit ventrad with the vestibulum oris has already been mentioned, as has also the insertion 

 of the superficial division of the masseter into its aponeurosis. 



The apical region of the orbit between the frontal arch and the infraorbital ledge is closed 

 by the orbito-sphenoid and a portion of the presphenoid. These arches form together an oval 

 with its long axis obliquely directed caudad and ventrad. In this aperture appears above a 

 broad flat surface of orbito-sphenoid. Caudo-ventrad to this is the large sphenoidal fissure, 

 at the ventro-rostral margin of which appears a portion of the presphenoid. This is separated 

 by a fibrous interval from the external pterygoid. The optic foramen pierces the orbito-sphenoid 

 below its middle, and is connected by a narrow cleft in the cartilage with the sphenoidal fissure, 

 from the contents of which the optic nerve is separated by a broad partition of connective tissue. 

 The sphenoidal fissure in addition to its usual contents, transmits the maxillary division of the 

 fifth nerve. 



Cranial cavity. The long axis, extending from the foramen magnum to the dorsal margin 

 of the frontal ossification centre, is approximately parallel to the basis cranii and has a length 

 of 49 mm. The greatest dorso-ventral diameter, between the rostral margin of the supra- 

 occipital and the sella turcica is 29 mm., and the greatest transverse, which falls in the same 

 plane, is 40 mm. Caudad of this transverse plane the cavity narrows like a funnel to the foramen 

 magnum, while rostrad it is rounded and broadly concave in the frontal region. The floor is 



