THE FURBISH LOUSEWORT may be a lovely plant, if you like scraggly snapdragons. And the snail darter may be more delightful than the average three-inch fish. But something is awary when a clump of louseworts along the Upper St. John River can louse up planning for the Dickey-Lincoln Dam - or when a federal court, to save the snail darter, stops the nearly-complete Tellico Dam down on the Little Tennessee River.

Misty-eyed environmentalists are delighted to see such obsure bits of nature hold sway over huge public works. They are also coming to regard the endangered species act as a weapon of last resort against projects that they oppose on broader grounds. The more pragmatic dam-fighters recognize, however, that many more snail-darter-type showdowns or more lousewort jokes can endangers the law itself. Already some members of Congress are grumbling that when they approved the act, they had in mind good casues such as saving bald eagles and keeping commercial foragers from ripping off great act in the West. They didn't mean to give automatic priority to a whole assortment of undistinguished flora and fauna with precarious existences and funny names.

What Congress should bear in mind as it considers changes in the law is that in almost all of perhaps 200 cases where endangered species and some project have seemed to collide, a means of coexistence has been found. Often all that's required is some care and redesign. Down on the Gulf Coast, after a great brouhaha, the last habitat of the Mississippi sandhill crane may be preserved by rerouting Interstate 10. In California several types of butterflies may be saved by setting aside some small preserves, including parts of an oil refinery, Los Angeles airport and an Army rifle range. Some species, too, are more adaptable than you might think. One rare butterfuly is hanging on amid the television towers of Twin Peaks in San Francisco, while some endangered birds are reported to be thriving along various freeway shoulders and medians.

The Tellico case is the first in which a choice seems to be unavoidable.Supporters of the project want Congress to exempt it from the law. Some opponents welcome hearings as one more chance to advance all of their arguments againt finishing the dam.By taking that tack they are acknowledging that, in the rare instances in which accommodations cannot be worked out, a project should not be canceled just because of one endangered fish or flower. We have not reached a conclusion about the Tellico dam on its merits. But we do think the decision should not be dictated by the snail darter along. The same applies to the lousewort and the studies of the Dickey-Lincoln project that are now under way.