I see you are now following through father John from this post
www.rootschat.com/forum/index.php/topic,366011.0.html 
IGI have William's birth earlier:
WILLIAM DOUGLAS SHENNAN Birth: 20 SEP 1855 Borgue, Kirkcudbright
Parents:JOHN SHENNAN and MARY MCKEAN
As you can see, mother Mary's surname is slightly different also. There are no births showing after William's in 1855 and nothing before on the Old Parish Registers but this looks like the family in 1861:
John Shennan 38, farm servant, b. Twynholm, Kirkcudbright
Mary Shennan 40, b. Twynholm, Kirkcudbright
David Shennan 10, b. Borgue, Kirkcudbright
Robert Shennan 8, b. Borgue, Kirkcudbright
William Shennan 6 , b. Borgue, Kirkcudbright
Address: Old Blackcraig, Borgue, Kirkcudbright
William's birth is an 1855 entry, the first year of official registration in Scotland. For one year only the entries ran over two pages of the register. For births it should also include a lot of details on the parents: ages, where they were born, date and place of marriage, how many children they'd had (boys/girls, alive/deceased). Worth your while looking at that image on line at
www.scotlandspeople.gov.ukMonica
