Hook: Mustad 94845 or 94840
Wings: Grizzly hen hackle tips
Tail: Brown and grizzly hackle
Body: Muskrat fur
Hackle: Brown and Grizzly

I know, it's not really a Catskill pattern. It was developed in Michigan as a Caddis pattern by Leonard Halladay in the early 1900s. If you asked 10 fly fisherman to name a Catskill MAYFLY pattern, it's my guess that over half would name the Adams. That makes it a defacto Catskill fly in my book. This is perhaps the most versatile dry fly pattern ever invented.