Mesolimulus walchi
Jurassic
Solnhofen Limestone
Eichstadt, Germany

Excellent 3 inch horseshoe crab on 6 inch x 8 inch squared slab

Horseshoe crabs are not a type of crab nor other crustacean.  They are actually more closely related to spiders and scorpions.  Here is a quite displayable specimen centered on matrix.