Under the Clayton Act (15 U.S.  Code § 18), certain business acquisitions are prohibited where “the effect of such acquisition may be substantially to lessen competition, or to tend to create a monopoly.” Long-standing jurisprudence has established that merger challenges require, at the outset, a prima facie showing of the likelihood of a substantial lessoning of competition that would result from the merger or acquisition.  Such prima facie showing typically takes the form of claims and evidence related to market shares above a certain level, but can take other forms.