1 66 THE AMERICAN MIDLAND NATURALIST 



proposes in this paper to associate as far as possible with the sex 

 of the shells examined. 



The material used was Dr. A. E. Ortmann's splendid collection 

 of shells in the Carnegie Museum at Pittsburgh, Pa., and while 

 the most complete account of how these measurements were 

 taken is reserved until the publication of the research spoken of, 

 the method of making them is described to what is believed to be 

 a comprehensible extent in the accompaning table dealing with 

 sexual dimorphism. Here it may suffice to state that the dimen- 

 sions taken were 7 in number and included the measurement of, 



I. — Relative degree of inflation. 



2. — Height. 



3. — Posterior length of shell. 



4. — Anterior length of shell. 



5. — Length of posterior hinge line. 



6. — Length of anterior, hinge line. 



7. — Thickness of shell — in this investigation taken just 

 superior to the pallial line in the region directly beneath the umbo 



In making these measurements an ordinary metric caliper 

 and rule were used, the values obtained reduced to convenient 

 factors by division into the length, with the exception of that of 

 thickness, which it seemed desirable to compare with the height. 



Measurements were made on the following genera and species, 

 viz., 



Fusconija flava, Raj. Paraptera fragilis, Raf. 



Amblema costata, Raf. ' Proptcra alata, Say. 



Pleurobema ohliquum coccinetm\,Con. Anodontoides ferussacianiis ,Lea 

 Elliptio dilatatus, Raf. Eurynia recta latissima, Raf. 



Symphynota costata, Raf. ' Lampsilis luteola, Lam. 



Anodonta grandis, Say. Lampilis ovata ventricosa, Lam. 



The factors above described having been obtained, it was the 

 practice while making comparisons in the endeavor to associate 

 any of the 7 measured morphological dimensions with the sex of 

 the shell, to compare equal numbers of both sexes of the species. 

 An average was made of the values obtained by calculation for 

 each dimension of the shell, and then this result compared with 

 that similarly obtained from the other sex of the animal. A table (I) 

 showing the averages for each dimension of that sex of each species 

 is appended, and from the comparison of its values, the table on 

 Sexual Dimorphism (II) is obtained. 



