Ophioglossum

1. Leaves gradually tapered to the base, elliptic in outline (widest ± at the middle).

O. pusillum

1. Leaves abruptly tapered to the base, ovate in outline (clearly widest below the middle).

O. vulgatum

All species found in Ophioglossum

Ophioglossum pusillumNORTHERN ADDER'S-TONGUE 
Ophioglossum vulgatumSOUTHERN ADDER'S-TONGUE 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. March 29, 2017. http://michiganflora.net/genus.aspx?id=Ophioglossum.