Complex sentence: Polly's mother is never mentioned and is presumably deceased, though Professor Spark's mother (Polly's paternal grandmother) is and she is described, by Polly, as cooking "odoriferous" sausages.
Split into simpler sentences: Polly's mother is never mentioned and is presumably deceased. However, Professor Spark's mother (Polly's paternal grandmother) is mentioned and is described, by Polly, as cooking "odoriferous" sausages.