A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form.
In this sentence, I cannot understand it very well. So, here is what I comprehension of this sentence, a smaller shell opening restricts mobility and ingestions, and the mobility and ingestion of a snail of the commoner form does not restrict or less restrict, am I right?
The reason you cannot easily understand that is because it is not a sentence. It is a phrase, and it is missing a vital element to make it understandable.