This is a reasonable question.

This paper can be downloaded from

https://www.zentralblatt-math.org/jahrbuch/?q=au%3Ade%20Longchamps%2C%20G%2A

See, also:

http://serge.mehl.free.fr/anx/bicorne.html

I was mistaken in what I wrote earlier.    The Zentralblatt math URL gives the review of the desired article but not the article itself. 

Also, the complete citation for the article by G. de Longchamps is

de Longchamps, Gaston Albert Gohierre, 1897. Note sur le bicorne, Journal des math´ematiques sp´eciales 21, 35–41.

A good paper about de Longchamps' work is

https://mediatum.ub.tum.de/doc/1281075/1281075.pdf

This is a great paper on geometry.   It includes a very interesting section on Riemann surfaces (see, e.g., page 92).

I am still looking for the missing paper. 

Maybe here?

http://onlinebooks.library.upenn.edu/webbin/serial?id=jmathelemfr