RESULTS , 305 



shell has a differing size index for different stages of its own individual 

 growth; and, secondly, because size of specimens of any particular 

 faunule is determined partly by the stage in individual growth at which 

 the specimen died, but chiefly by the rate of motion of the currents 

 which selected the particular shells at the point of their fossilization. 



2. Size of muscular scar — Values of index R^. — (2a) The average of 

 E^ for 142 specimens for all zones is E^ = 64.1.- 

 (2&) The average for the specimens of each zone by zones: For 12 

 Hamilton specimens is 60.7; for the Ithaca, no evidence, 0; for 48 Van 

 Etten specimens is 64.8 ; for 26 Owego specimens is 63.2 ; for 56 Swart- 

 wood specimens is 64.7. 



(2c) In order to catch more fully the ontogenic law of development 

 of this character, it is necessary to note the fluctuation in value of the 

 index E^. 



For the Hamilton set the amount of fluctuation is from 47 to 71 per 

 cent = 24 per cent. 



For the Van Etten set the amount of fluctuation is from 57 to 82 per 

 cent = 25 per cent. 



For the Owego set the amount of fluctuation is from 56 to 72 per 

 cent = 16 per cent. 



For the Swartwood set the amount of fluctuation is from 52 to 78 per 

 cent = 26 per cent. 



Phylogenic increase in size of muscular scar. — This shows a slight 

 phylogenic increase in proportionate size of the muscular scar, both in 

 the lower and higher limits, namely, from 47 to 52 and from 71 to 78. 



Examining this character for one of the earlier representatives of the 

 genus, namely, Orthis hybrida Hall of the Niagara, we find in the type 

 figures the index E^ = 56. Two characteristic Coal Measure forms at 

 the other extreme of the evolutional history of the genus give E- = 63.3 

 (for Orthis pecosi Marcou) and E^ = 70 (for Orthis penniana Derby) ; 

 difference, 56-70 = 14 per cent. This shows that the phylogenic ten- 

 dency is decidedly toward an increase in the length of the muscular scar 

 with time, represented by 14 per cent between the Silurian and Car- 

 boniferous samples. 



Fluctuating value of index R^ for three type species of the Hamilton. — 

 Again, if we examine the type specimens figured to represent the several 

 Hamilton species, the fact appears that Orthis leucosia may normally vary 

 in value of E^ from 69.2 (figure 4/?) to 83.9 (figure 4t), or 14.7 per 



* The number 64.1, as the value of R^. means that the muscular scar averages 64 and 

 1/10 per cent of the length of the shell, and so in all expressions of the value of R^. 



