The Collaborative International Dictionary
Seldom \Sel"dom\ (s[e^]l"d[u^]m), adv. [Usually, Compar. More seldom (m[=o]r" s[e^]l"d[u^]m); superl. Most seldom (m[=o]st" s[e^]l"d[u^]m); but sometimes also, Seldomer (s[e^]l"d[u^]m*[~e]r), Seldomest.] [AS. seldan, seldon, seldum, fr. seld rare; akin to OFries. sielden, D. zelden, G. selten, OHG. seltan, Icel. sjaldan, Dan. sielden, Sw. s["a]llan, Goth. sildaleiks marvelous.] Rarely; not often; not frequently.
Wisdom and youth are seldom joined in one.
--Hooker.
Wiktionary
adv. (en-comparative of seldom POS=adverb)
Usage examples of "seldomer".
On the second day it was observed seldomer, and was therefore not seen to change its course so often, viz., only 6 times, but in the same complex manner as before.
Women, it is said, seldomer fall under the penal law -- contribute a much smaller number of offenders to the criminal calendar, than men.