360 University of California Publications in Zoologij [Vol. 21 



in which the teeth develop occurs below the level of the palate instead 

 of above it. A purely mechanical explanation might be that the grow- 

 ing eye constitutes a stronger barrier to growth of the maxilla upward 

 than the buccal cavity constitutes to its growth downward. Many 

 other factors much more complex in their effect probably help to 

 determine the direction of growth. In A the zygomatic portion of 

 the maxilla is weak and does not extend laterally beyond the outer 

 margin of the infraorbital canal. In 5 a slight swelling marks the 

 strong crest that, in adults, develops along the anterior portion of the 

 zygomatic root. This is only slightly better developed in D than in A. 

 The plate that is developed below this ridge and that faces ventrally 

 in adults is part of the region where the masseter muscle has its origin. 

 This plate is present in G as an oblique surface inclined outward and 

 upward and at this time projects only 1 mm. laterally beyond the 

 external border of the infraorbital foramen. The plate develops 

 rapidly from H on and becomes modified in progressively older skulls 

 until the condition in Q is reached. Here the sharp overhanging crest 

 with a distinct lobe has a deep fossa beneath it. Coincident with these 

 changes the zygomatic root of the max;illa becomes inclined outward 

 at a wider angle from the longitudinal axis of the skull to permit of 

 the zygomatic root taking part in the bowing outward of the whole 

 zygomatic arch. The maxillar;/ plate that faces ventrally and is 

 developed beneath the zygomatic ridge develops very little until a 

 sufficient number of the tcetli, \, \, f> T' <^^^^Pt from the gums to 

 make possible the mastication of solid food. Apparently, the develop- 

 ment of this plate depends upon the use and growth of the masseter 

 muscle which ha.s its origin partly on this plate. The zygomatic 

 ridge extends to, but not beyond, the premaxillo-maxillary suture. 

 Just behind the last molar and between the maxilla and the palatine, 

 there is a notch for the transmission of a nerve. In E the maxilla 

 sends back, lateral to the notch, a thick process which meets the palatine 

 bone and converts the notch into a canal. The process of the maxilla 

 does not at once fuse with the jialatine but grows thinner in suc- 

 cessively older stages until is reached. In and in later stages this 

 process of the maxilla is sometimes fused with the palatine, sometimes 

 in contact with but not fused with the palatine, and sometimes it 

 remains wide open as a mere notch. Measurements taken along the 

 intermaxillary suture show a postnatal increase in length of 219 

 ])er cent. 



Lacrymals. — Each lacrymal is ossified in B except for tlu^ postero 

 ventral portion, which has become completely ossified in F. In B a 



