1926] Hall: Slull of the Eodeiit Otospermophilus grammurus beccJtei/i 



403 



The greater degrees of development of all small and large ridges, 

 crests, and processes with increased ag'e are among the main features 

 of change with growth to be noted in the bones of the skull of either 

 sex at so late a period as the bulk of these 445 skulls represent. The 

 following comparison of the two age groups indicates the nature of 

 these and other changes. 



Criteria for Determining Age Groups of Males 



Less than one year old 



Median occipital keel weak or want- 

 ing. 



Lambdoidal crest projecting only 

 slightly posteriorl}\ 



Sagittal crest not extending more 

 than one-third of distance from 

 lambdoidal crest to line drawn 

 between posterior margins of the 

 bases of the postorbital processes. 



Parietal ridges low and bowed out- 

 ward. 



Postorbital processes with distinct 

 suture between frontal and parietal 

 elemeiits. Light. 



Sharp V-shaped angle at junction of 

 postorbital process and parietal 

 bone in posterior view. 



Supraorbital ridge thin, 

 bital notch open. 



Supraor- 



Transverse A^entral ridge of jugal 

 at point of anterior dorsal flexure 

 weak or wanting. 



Reflexed margin of zygomatic root 

 portion of maxillary plate straight. 



Anterior palatine foramina widest 

 posteriorly. 



Premolars not worn. 



Occipito-nasal length not usually over 

 57 mm. 



Mo7-e than one year old 



Median occipital keel strongly de- 

 veloped. 



Lambdoidal crest pushed far poste- 

 riorly at median portion of either 

 lateral half. 



Sagittal crest extending more than 

 one-third of distance from lamb- 

 doidal crest to line drawn be- 

 tween posterior margins of the 

 l)ases of the postorbital processes. 



Parietal ridges high and straight. 



Postorbital processes with frontal 

 and parietal elements fused. 

 Heavy. 



U-shaped indentation at junction of 

 postorbital process and parietal 

 bone in posterior view. 



Supraorbital ridge thick, heavy. 

 Supraorbital notch constricted 

 and often a canal. 



Transverse ventral ridge of jugal at 

 point of anterior dorsal flexure 

 strong. 



Reflexed margin of zygomatic root 

 portion of maxillary plate with 

 well developed lobe. 



Anterior palatine foramina constricted 

 posteriorly, being slit-like or widest 

 at middle. 



Fourth premolars worn. 



Occipito-nasal length usually over 

 58 mm. 



