Final answer:
The fallacy present in the quoted Dred Scott v. Sandford passage is an ad populum fallacy, which bases its reasoning on common belief rather than factual evidence.
Step-by-step explanation:
The passage from the Dred Scott v. Sandford Supreme Court decision contains the fallacy known as ad populum, which appeals to popular opinion or the majority belief to argue that a concept is true or acceptable. In this case, the decision's reasoning relies on the supposed common consent that the 'negro race' was excluded from 'civilized Governments and the family of nations' and considered slaves. This type of argument assumes that just because a belief is widely held, it is inherently true, without providing actual evidence to support the claim.