Slytherin Purebloods?

[justcarol67]

No: HPFGUIDX 88174

Someone (identity snipped from post) asked:
> >Many people who theorise about the Harry Potter universe and who
> >write fan fictions seem to assume that students who get sorted into
> >Slytherin must necessarily be purebloods; likewise they assume that
> >Snape must be a pureblood, too. But can they be? It seems Slytherin
> >House would have to be the smallest of all houses - as far as we know
> >from the books, there are less than ten pureblood families left in
> >all. I would guess that most wizarding families have Muggles
> >somewhere in their ancestry, and that being against 'Mudbloods' does
> >not necessarily preclude one's being a pureblood oneself. In the days
> >of Salazar Slytherin it might still have been possible to select
> >Slytherins on blood basis only, but surely it cannot be the case
> >anymore now?
>
Troels answered: 
 There are, and has been, several half-blood wizards in Slytherin.
>     Tom Riddle, Millicent Bulstrode and Tracey Davis.


Tracey Davis? I can't remember encountering the name. Where is it
mentioned and how do you know that he/she is a "mudblood"? And can you
tell me where Millicent Bulstrode's parentage is mentioned? All I
remember about her is that her cat was the source of an unpleasant
experience for Hermione in the polyjuice incident in CoS. It seems odd
that Pansy Parkinson would tolerate Millicent's presence in her
dormitory she knew she was a "mudblood."

Carol