| "Connected" means "connected by edges", rotations and reflections are not considered different, but the internal arrangement of the dominoes does matter.
I have verified the first three entries by hand. The terms 255 and 2874 were taken from the Vicher web page. - N. J. A. Sloane (njas(AT)research.att.com).
In the bottom half of page 2 of his rough sketches ... N. J. A. Sloane made two errors which canceled each other out: he mistakenly duplicated the third figure in the first column (middle figure in the second column is 180deg rotation), and omitted the stack of three placed horizontally with the middle piece shifted one unit to the right. The count of seven in that group remains correct. [From Owen Whitby (whitbyo(AT)acm.org), Nov 18 2009]
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