192(i] HaU: Skull of the Rodent Otosi)ermop]ii1us (/rammunis heecheyi 359 



posterior margins of the alveoli of the incisors in B (fig. 19) lie slightly 

 anterior to the anterior palatine foramina and less lateral to them than 

 in A. The distance gradually becomes greater between these foramina 

 and alveoli. In A each premaxilla consists of two ossified portions (fig. 

 20). An iinossified area extends diagonally back from the posterior 

 border of the alveolus of the incisor to the maxillary border of the pre- 

 maxilla. Traces of this division between the two parts of each pre- 

 maxilla are also present in B. On the ventral side of the rostrum 

 the premaxillae increase 3i^ times in length during postnatal develop- 

 ment. The lateral wings of the premaxillae that meet the frontals 

 vary greatly in the degree of posterior extension in different specimens 

 of the same age. These wings sometimes extend farther posteriorly 

 on the frontals, than do the nasals, and sometimes not so far. The 

 increase in length of these two wings of the premaxillae is equal to the 

 increase in length of the nasals. The postnatal increase in width of 

 the two wings, 469 per cent, is more than the increase in width of the 

 nasals, which is only 200 per cent. In E two slight depressions appear, 

 one just behind each incisor tooth, on the ventral face of the pre- 

 maxillae. These gradually deepen until the condition shown in Q 

 is developed. 



Maxillae. — In A the maxillae are ossified and are in contact with 

 ossified portions of the frontals, jugals, and premaxillae. In B the 

 maxillae come into contact with the lacrymals and palatines. The 

 infraorbital canal remains constant in position, as does the process 

 for muscular attachment immediately below it. The palatal surface 

 is nearly flat in A to C, concave in D to K, and develops a median 

 ridge in L which becomes most pronounced in Q. The distance betw^een 

 the first molars (measured in the young skulls from the median edges 

 of the dental troughs) increases only 90 per cent from C to Q. From 

 F to Q there is an actual decrease of 5 per cent. From H, where this 

 distance reaches its maximum, to Q there is a decrease of nearly 16 

 per cent. The concave palatal surface of the maxillae is present at a 

 time, D to K, when the teeth are pushing down the regions of the 

 maxillae that they occupy; thus the concave palatal surface of the 

 maxillae seems to be due, in part at least, to the lateral borders being 

 pushed down rather than to the center of the palatal region being 

 pushed up. This concavity is greatest in E, just before the teeth begin 

 to break through the gums, and disappears in K, the stage in which 

 the last molar comes into use. These parts of the maxillae in which 

 the teeth develop are bounded above by the eye and below by the 

 buccal cavity. The increase in thickness of that portion of the maxillae 



