PLASTICITY AND PERMANENCY OF CHARACTERS. 309 



mens showing all grades of modification, from one or more to 

 what might be represented by number six, on a scale of ten ; 

 and in plicatella, the variation is one to four. In some 

 later forms the variation for each species is slight, rarely more 

 than one or two tenths, using this means of designating the 

 degree of plasticity. In the Spirifer Icevis, found abundantly 

 in the rocks at the foot of Fall Creek, Ithaca, N. Y., there 

 are generally no plications, but occasionally a specimen is 

 found with the margin for half an inch up corrugated by this 

 modification. In this species the character is probably the 

 remnant of a plasticity more strongly expressed in its an- 

 cestors. 



The general development in number of plications is noted 

 on some lines of species, especially those showing bifurcation 

 of the plications during growth ; and, as in the case of the 

 median fold and sinus, this character is developed in the two 

 directions of increase and decrease, in different races. 



In one series increase, by dichotomy of the surface plica- 

 tions, beginning in adult forms and becoming more and more 

 early in starting, affects first the centre of the shell, then the 

 neighboring parts of the side until the whole surface is 

 affected, but by slow degrees; so that, expressing the evolu- 

 tion in the same way as heretofore, the rate of development 

 is approximately as follows: 1-3 in Lower Devonian, 2-4 in 

 Upper Devonian, 3-7 in Lower Carboniferous, 6-10 in Upper 

 Carboniferous. 



This modification appears to be dependent upon, or ex- 

 pressive of, the rate of increase of the shell in either the 

 radial or in the circumferential directions. If the circumfer- 

 ence of the shell increases more rapidly than the growth in a 

 radial direction the margin becomes too large for the shell at 

 its normal distance from the beak, and it is necessarily puck- 

 ered into folds to accommodate itself to its conditions; thus 

 as it grows its surface becomes plicated into folds. When the 

 growth in the radial direction keeps up with the increase in 

 the circumferential direction the shell remains smooth, and no 

 plications are developed. Thus the increase in the number of 

 plications for a given shape of shell is evidently due to the 

 acceleration or earlier starting of the differentially excessive 



