Mucrospirifer is so named due to it's "mucronate" appearance (mucronate is an adjective that means "ending abruptly in a short sharp point or 'mucro'"). That description is based on specimens that have the margin of the shell meeting the hinge line at an acute angle rather than a rounded appearance.The specimen below illustrates that habit well on the right side of the specimen.
But it seems nearly all of the above species were recombined in 1964 by John Tillman ("Variation in Species of Mucrospirifer from Middle Devonian Rocks of Michigan, Ontario, and Ohio", John R. Tillman, Journal of Paleontology, Vol. 38, No. 5 (Sep., 1964), pp. 952-964) into M. mucronatus with the exception of M. thedfordensis. Tillman remarked that "M. thedfordensis (Shimer & Grabau)
differs from M. mucronatus (Conrad) in having a U-shaped to V-shaped sulcus, never with a flattened floor; a low moderately convex fold, never with flattened surface."
Finally, here is an example of M. mucronatus from the Silica shale.
This specimen was found at a borrow pit on Deep Springs Road in Madison County, NY which exposes the Windom shale member of the Moscow formation (middle Devonian in age, Givetian stage.)