Tony, I think it's square I'll have to look at it.. I gave the ski to my buddy to get running he broke off one of the gas line tubes on the carb so he drilled it out and cracked the housing then tossed the carb in the trash ! But I believe it's a square bottom mount .. The ski is a 91 Waverunner 650 LX, It is a clean ski ! I had it running before the carb issue and sounded good.. Picked up the ski with a good galvanized trailer for 400.00