This may consist in mere transposition of the vessels, as when we find an arteria innominata on the left side, and the right carotid and subclavian arising separately from the arch without any other transposition. This is very rare.

* " Lecons d'Anatomie Comparee," tome iv. p. 249. Varieties of the Arteries, p. 7.

The two carotids may arise between the subclavians by a common trunk. This is the regular disposition in the elephant.

There may be a common trunk for the two carotids ; on the left side of this may be the origin of the left subclavian; and at the extremity of the arch, the origin of the right subclavian.

Lastly, there may be an arteria innominata on the right side for the right subclavian, and the two common carotids; on the left side of this, the origin of the left vertebral; and still more to the left, the origin of the left subclavian.