Grier: Morphological Features of Mussel-shells. 173 



4. Correspondingly, the posterior hinge-length (PHL) tends to 

 have a greater development in Lake Erie, and, as observed in the 

 table, is apparently positively correlated with the posterior develop- 

 ment of the shell. The converse appears to be established, i.e., that 

 the anterior hinge-length is best developed in the Upper Ohio, etc., 

 and also positively correlated with the greater anterior development 

 of the shell. 



5. Thickness, as a rule, is greater outside of Lake Erie. 



We have seen that the ancestors of the Lake Erie and Upper Ohio 

 forms were derived from the same stock. Since some of the latter 

 went over into the lake and became modified, the problem this in- 

 vestigation tries to answer is "How have they changed?" The 

 anszver is given in the preceding paragraphs. If we put a shell in the 

 lake environment we may expect it will change its morphological 

 features, not at random, but in a distinct, determinate, or orthogenetic 

 direction, as is now seen by the fact that other shells behave the 

 same way. 



IX. Correlations. 



From Tables W and V also the following correlations may be 

 derived in the case of shells from Lake Erie: 



1. A decided tendency toward a greater degree of inflation {DSD) 

 width, associated with less dorso-ventral diameter {DVD) height. 



2. In all but two species (to which there are exceptions) there is a 

 greater degree of inflation associated with greater posterior develop- 

 ment of the shell {PD). 



3. In all but two species greater posterior diameter is associated 

 with greater posterior hinge-length. 



4. Greater DSD, PD, PHL, and less DVD are associated with less 

 thickness. 



5. These dimensions correlate with one another throughout the 

 lake environment. 



With regard to the Upper Ohio shells the following seems true, viz.: 



1. A decided tendency toward a less degree of inflation associated 

 with a greater height and length of shell. 



2. Less degree of inflation with a greater anterior development. 



3. In most species, greater anterior development with greater 

 length of anterior hinge-line. 



4. Less DSD, greater DVD, AD, AHL, associated with greater 

 thickness. 



