Buriallusion: Phantomic Killer in Night Mist
I do believe the Shellder has been dealt with, so I'll just explain egg moves (EMs) very briefly.LF Skill Link on Shellder and a basic explanation on egg moves? Because they confuse me and it makes no sense as to why my Larvitar kept its egg moves but my Snivy didn't :/ Either a brief rundown of how they work or a pointer to a guide on-site is what I'm looking for :)
In Gen VI+, both male and female parents (assuming non-Ditto) are able to pass EMs to the children. Priority-wise, the female passes its EMs then the male.
In Gen V and below, however, only the male can pass EMs.
Of course, only actually compatible EMs will pass, refer to Bulbapedia or Serebii for the list of EMs for any given Pokemon.
In your case, assuming this is Gen VII, the only reason I can think of is if your Snivy parents have a combined total of 8 EMs (or at least 5 or more) and the female's passed and the male's did not. It might be another reason, but I cannot think of one at this present moment.