Okay Wen let me see what I can do to help.
First part of the problem is that there were two Selina Tonkins baptised within 11 months of each other.
This is the one Carol found arriving on the 'Gipsy Bride':-
Selina Tonkin bp. 31st October 1841 at St Levan daughter of John and Eliza Tonkin of Trevean
According to the Victorian Great War Index Selina Jeffrey died in 1920 age 80 at Hawthorn so, as you say, she was born about 1840.
Her father is recorded as Jas. and mother Unknown but this can often be misleading.
However the Index to South Australian marriages shows that Selina's father was James so there can be little doubt!
In this case however it appears to be correct and this will be your Selina:-
Selina Tonkin baptised 1st November 1840 at St Buryan daughter of James (labourer) and Elizabeth Tonkin of Church Town
In the 1841 Census James and his family were living at Trenuggo, Sancreed and Celenia (Selina) was age 7 months.
James TONKIN, 42
Elizabeth do., 44
Mary do., 13
Martha do., 12
Honnor do., 6
Celenia do., 7 mths
Let me try to compile a probable list of children:-
Mary bp. 6th April 1828 St Buryan d/o James (labourer) and Elizabeth Tonkin of New Town
Martha bp. 15th August 1830 St Buryan d/o James (labourer) and Elizabeth Tonkin of Boscawen
Honor bp. 27th July 1834 St Buryan d/o James (labourer) and Elizabeth Tonkin of West Moore
Selina bp. 1st November 1840 St Buryan d/o James (labourer) and Elizabeth Tonkin of Church Town
Other possible children:-
Elizabeth bp. 15th January 1826 St Buryan d/o James (labourer) and Elizabeth Tonkin of Church Town
James bp. 12th September 1832 St Buryan s/o James (labourer) and Elizabeth Tonkin of Clapper
There was more than one other family of James Tonkin with a wife Elizabeth so things do get a little confusing but I should think it likely these last two children belong as siblings to Selina.
Confusing matters more was an illegitimate child named James Tonkin Matthews baptised in 1826 son of Elizabeth Matthews. (James Tonkin recorded as father.)
But it begins to look like the following marriage might be the one you would be interested in:-
James Tonkin, labourer of St Buryan, married Elizabeth RUSSEL, spinster of St Buryan, 17th July 1824 at St Buryan by Banns.
Witnesses were Nicholas Turner and William Hutchens
This is one of a large number of marriages at St Buryan where the term 'with consent of parents' was used.
However I know that many of these marriages did not involve minors.
Searching South Australian Passenger Lists from
www.familyhistorysa.info/shipping/passengerlists.htmlhas produced the follosing possibility:-
TONKIN James, wife (Ann/Eliz?), William Doble, Jas, 3 ch inc Selina (1 d@sea) arrived 1847-08-19 on Cressy from London 1847-05-14 [Source:6,7,10,20]
I now need to see if I can find anything more that might establish whether we have the right Selina.
At least we know the father is James and the ship arrived in South Australia.
I will post any further results here.
CT