LCM3 Landing Craft 

The LCM (also called a “Mike boat”) was the most widely used type of tank-carrying landing craft in WWII and more than 8,500 were built in American yards between 1942 and 1945. LCMs could be either carried on ship davits, or towed across the English Channel. 

Due to the danger of swamping in rough seas, most LCMs were towed across the Channel and not loaded with troops until the mornings of D-Day. LCMs could be readily identified by their unique perforated bow ramp. They could also handle up to 30 tons of cargo or 60 troops (twice more than a LCVP. LCM (3) had a crew of 4 men and was armed with two 50-caliber MG. 

 The most important role played by LCMs on D-Day was transporting the engineer teams assigned to demolish the beach obstacles. 

Source:

U.S. Marine Corps World War II Order of Battle by Gordon L. Rottman

The battle of Normandy