Ground beetles in the genus Pterostichus, subgenus Euferonia, are common woodland beetles in the eastern U.S. Almost all have reduced wings, so are unable to fly.
I’ve collected a number of specimens that fit into this species pair, but have struggled with separating them with existing keys. Lindroth noted that “the infraspecific variation of external characters is considerable” and has resulted in many unnecessary names, especially by Casey.
The characters that separate P. stygicus from P. coracinus are the presence of a “tubercle” in the basal fovea of the pronotum and the elytra usually being somewhat iridescent. The iridescence isn’t hard to see, but the “tubercle” is very subjective. It appears to be referring to a low ridge or convexity within the fovae, hardly tuberculate in my opinion, and quite variable. Fortunately, I recently collected a pair of males and decided to consult Lindroth, who illustrates the genitalia of the species. The shape of the penis is very distinct between the two species: symmetrical in P. stygicus and asymmetical in P. coracinus. This led to the reevaluation my previously collected specimens, which turned out to be all P. stygicus.
Now I just need to find P. coracinus!