378 University of CaJifoniia Publications in Zoology [Vol.21 



ridge, which separates them, develops. In adults, near the anterior end 

 of the basioccipital, two processes, one on each side, extend ventraJly 

 as far as do the tympanic bnllae. These are closely appressed to the 

 sides of the tympanic bullae. This relation gives a broad support to 

 the basioccipital and shields the bullae from the disturbing effects 

 of muscular action. While these processes eventually grow as far 

 ventrally as do the bullae the two structures by no means keep pace 

 with each other in development. The bullae are well developed in 

 F, but only the first traces of these processes of the basioccipital are 

 present at this time. In K the processes are one half the height of 

 the bullae. In Q the bullae and lateral basioccipital processes are 

 equal in height. The posterior margin of the basioccipital is- thick- 

 ened. Part of this thickened border goes to form the ventro-mesial 

 parts of the occipital condyles. The superior surface of the basi- 

 occipital is concave but smooth. The two exoccipitals fuse M'ith the 

 basioccipital in L. During post-natal development the basioccipital 

 increases from 3.0 to 12.2 mm., or 307 per cent in length, and from 

 2.7 to 9.3 mm., or 244 per cent in M'idth. 



Exoccipitals. — In A these bones are present as two small ossified 

 areas entirely surrounded by cartilage. The portions bordering on 

 the foramen magnum that will later form the larger part of the 

 exoccipital condyles are slightly thicker than the rest of the bones. 

 In C contact is made with the basi- and supraoccipital. Contact is 

 made with the tympanic in C. In F, when the mastoids are ossified, 

 these bones and the exoccipitals grow downward and form the par- 

 occipital processes. At this time the paroccipital processes are com- 

 posed almost Avholly of the exoccipitals, but with increase in age the 

 mastoids gradually contribute a larger share. In F the processes 

 taper to blunt points and project straight downward. They are not 

 appressed to the bullae. In G, H, and J, however, the processes are 

 closely applied to the bullae. Beginning with K continued growth 

 carries the processes away from the bullae. During that period of 

 growth from K onward, when the processes are growing dowuAvard, 

 they also curve forAvard and inward. In Q they project nearly as far 

 ventrally as a plane passing across the lower surfaces of the tympanic 

 bullae. On the external surface of each exoccipital two condylar 

 foramina are present. In several cases three foramina are present 

 on the internal surface, but two of these three unite within the body 

 of the bone; thus only two appear on the external surface. This is 

 a slight advance over the condition not infrequently })resent in Mar- 



