“A. Xylaria polymorpha group. Examples include X. anisopleura,
-X. bulbosa (PERS. : FR.) BERK. & BR.,- X. curta FR., X. feejeensis,
X. longipes NITS., X. obovata (BERK.) FR., X. polymorpha,
X. schweinitzii, X. tuberiformis, and probably, -X. castorea BERK.-
+ X. corniformis Some of these species can now be separated
on conidial and cultural features (ROGERS, 1983; 1984 a; 1984 b).”

-Rogers, J. D. 1985. Anamorphs of Xylaria: Taxonomic considerations. Sydowia 38:255-262. (PDF)

NOTE: X. bulbosa and X. castorea removed at Dr. Rogers’ direction, as USA material under these names are, in fact, X. corniformis

“Xylaria longipes  is  closely  related  to X. polymorpha,  from 
which  it  is  hardly  distinguishable  in  the  field,  both  species 
exhibiting  similar  highly  variable  stromatal  shape  and  corky‐
cracked  surface.  They  are  mainly  separated  based  on  different 
ascospore size range and  the conspicuously sigmoid germ slit of
X. longipes. Unlike what it is suggested by its name, the stromata 
of X. longipes are often short‐stipitate  to almost sessile.  It seems 
restricted  to  Acer pseudoplatanus, but  X. polymorpha also occurs 
on  this  substrate,  which  makes  microscopic  examination  of 
ascospores the only way to identify them safely.”

Fornier, Jacque. Update on European species of Xylaria, 2014. (PDF)

http://www.mushroomexpert.com/xylaria_polymorpha.html