This is the standard crank that comes on most modern unicycles. The hub axle has tapered ends with a square cross-section and a bolt or nut to hold the cranks on. The cranks are forced onto the axle to create a friction fit and locked in place with the bolt or nut. You should never ride with loose cotterless cranks as this will round the corners off the axle and distort the square hole in the crank, preventing them from fitting tightly ever again.
Splined cranks are stronger than cotterless cranks and therefore are more suitable for muni or trials unicycling, but require slightly more maintenance. The bolts will probably need to be tightened after a week of riding and checked regularly after that. For more information read our section on maintenance of splined cranks.