Vol. V] 



TAYLOR— NEW SUBGENUS OF PHENACOMYS 



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grouping. Of the series of longicaudus, No. 19985, a young 

 adult, has the tail 44.2 per cent of its total length. No. 19976, 

 also a young adult, is next, with the tail 42.5 per cent of total 

 length. The largest and oldest individual of all, No. 19984, 

 follows with tail 42.4 per cent total length. The smallest and 

 youngest example of longicaudus measured to date (No. 

 20657, preserved in alcohol) has tail only 25.7 per cent of total 

 length. Young animals are all characterized by low ratios, 

 and as the scale of age is ascended the ratio increases. 



Fig. 3— INCREASE IN RATIO OF LENGTH OF TAIL VERTEBRA 

 TO TOTAL LENGTH WITH AGE. 



The heavy black line represents the actual total length (correlated with 

 age) ; the light black line the ratio of the length of the tail vertebrae to total 

 length. Each round dot represents the average for six individuals. The pair 

 of dots on each vertical line pertain to the same six individuals. Read scale 

 for actual total length on the left side of the diagram, and scale for ratio of 

 the length of the tail vertebrae to total length on the right side. Note that 

 if the ratio of the length of the tail vertebras to total length remained 

 constant with increasing age at the figure for the smallest group, the 

 ratio would be indicated by a horizontal line, as dotted in the figure. 

 But instead of this, the ratio undergoes a steady increase with age. 



Of the other species of the genus, P. albipes makes the 

 nearest approach to P. longicaudus, the two known specimens 

 of this form having ratios of tail vertebras to total length of 

 36.9 and 40.0 per cent respectively. Three skins of P. inter- 



