Jurassic Park Wiki
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Mesolimulus is an extinct genus of arthropod. The best known examples are found in Solnhofen limestone near Solnhofen, Bavaria, Germany. Originally assigned to the living genus Limulus, they are related to and look virtually identical to the modern horseshoe crab.

Wikipedia has a more detailed and comprehensive article on Mesolimulus

Jurassic Park

Mesolimuulus has never appeared or mentioned in any Jurassic Park films or books, it has however appeared in Jurassic Park III: Park Builder.