1916] 



Taylor: Beavers of Western North America 



427 



Fig. C. Rostrum, dor- 

 sal aspect, to show 

 method of taking 

 measurement "Length 

 of nasals". About 

 one-half natural size. 



while the porcupine belongs to the section Hystriconiorpha. If the 

 above-mentioned resemblances are not due to inheritance from a 

 common ancestor, and the bulk of evidence 

 would seem to indicate that they are not, they 

 illustrate a noteworthy case of parallel de- 

 velopment in these widely different sections 

 of the Rodcntia. It is doubtless something 

 more than coincidence in this connection that 

 the beaver and the porcupine are bark-feed- 

 ers. The rounded instead of plane conforma- 

 tion of the palato-maxillary region, and the 

 similar complication of the enamel pattern of 

 the cheek-teeth, appear to be direct adapta- 

 tions: the first to the stripping of bark from 

 twigs and branches ; the second to its effective mastication. 



Dimensions of Crania 



From the table of cranial characters (opp. p. 426) it appears that 

 with age: (a) nearly every bone increases in size, each outside skull 

 dimension becoming greater; (6) the ratio of the length of the inter- 

 parietal to basilar length decreases (though no. 12101 departs rather 

 widely from this rule) ; (c) the mastoid width increases at prac- 

 tically the same rate as does the basilar length, although giving evi- 

 dence of a slight tendency to increase less rapidly. 



Temporal Ridges 



The degree of approximation of the temporal ridges is in general 

 indicative of age, although the rate of approximation may vary in 

 different forms. In leucodonta the temporal ridges first come together 



posteriorly, then continuously in 

 an anterior direction. In the 

 oldest skulls the ridges form a 

 sagittal crest, narrow posteriorly, 

 broader anteriorly, which ex- 

 tends to within 25 mm. of a line 

 drawn at right angles to the 

 antero-posterior axis of the skull 

 Lateral view of mandible, to a t the narrowest part of the 

 show method of taking interor bital constriction. 

 measurements. About one- 

 half natural size. Bailey (1905, p. 122) refers 



Fig. D. 



