274 ORTMANN— CORRELATION OF SHAPE AND 



Ohio have a diameter ahnost uniformly above 50 per cent, ; in the 

 region around Pittsburgh (in Ohio and Allegheny), it varies around 

 50 per cent. ; and in French Creek it falls below 45 per cent, (always 

 barring minor irregularities). 



Already this first instance substantiates the general law pro- 

 pounded in the introduction : in the larger rivers, these shells are 

 more convex and szvollen; in the headwaters, they are flat and conp- 

 pressed; and in the intermediate parts, the intergrades betzueen the 

 extremes are found. 



Under these circumstances it is, indeed, hard to draw a line 

 between the two forms called siihrotunda and kirtlandiana. Ac- 

 cording to measurements taken from Lea (Dia. 69 per cent.), there 

 are specimens which are even more obese than any of those ex- 

 amined by myself (max. 67 per cent.), and these swollen forms 

 differ very strikingly from those which have only 44 per cent, (as 

 in Lea's type of kirtlandiana), or even less (falling to 33 per cent, 

 according to Simpson and my material). In order to include politus 

 Say in the synonymy of subrotnnda, as Simpson has done, I have 

 decided to draw the line at the diameter of 50 per cent., so that indi- 

 viduals with this diameter and over shall be called Fnsconaia sub- 

 rotunda, and individuals with the diameter less than 50 per cent. 

 shall be called Fusmonaia subrotnnda kirtlandiana. Thus it is evi- 

 dent that two of the shells of subrotnnda, for which Simpson gives 

 the measurements, having the diameter 44 and 49 per cent., fall 

 under kirtlandiana. 



This will bring almost all specimens in the Ohio below Penn- 

 sylvania under the main species, while all specimens in French Creek 

 are the var. kirtlandiana. In the Pennsylvanian part of the Ohio, 

 and in the Allegheny, we have the two forms associated, sometimes 

 the one, sometimes the other prevailing, and the various sets from 

 these localities should be separated accordingly. This is only what 

 we should expect as the natural condition ; and also, for practical 

 purposes, this clears matters up, and brings sets together, which are 

 rather uniform. 



One additional circumstance should be pointed out. The obesity 

 of the shell changes a little zvith age, so that young shells, in the 



