This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
These varieties are exceedingly numerous. There may be a common trunk on the right side, giving off the right subclavian, and both carotids. This is the natural disposition in the simiae, and has been also observed in the dog, fox, wolf, lion, hyena, bear, and many other mammalia.
There may be a common trunk on the left side, giving off the left subclavian and both carotids. This is a much rarer variety than the preceding.
There may be two arteriae innominatae, one giving off the right common carotid and subclavian, and the other giving off the left common carotid and subclavian. This is the natural arrangement in the cheiroptera, and, according to Cuvier, occurs in the dolphin.*
Mr. Green remarks, "This tendency of the vessels towards the left side leads to an anomaly extremely rare, an example of which I have before me; in this variety all the vessels arise from the left side of the arch. First, the right carotid, which crosses the lower part of the trachea, giving off the right vertebral; next to this arose the left carotid and subclavian, nearly from the same point; the right subclavian is detached from the back part of the arch a little below the left subclavian: it passed to the right side, behind the oesophagus and trachea."†
Lastly, There may be two arteriae innominatae, one giving off the two carotids, and the other the two subclavians.