The horseshoe crab, shown at left,  is prolific and lives along the shores of the Atlantic Ocean from above the state of Maine southward along the Florida coasts.  The scientific name is Limulus polyphemus.  Horseshoe crabs are not really crabs.  They are "living fossils" and belong to a group of arthropods called CHELICERATES.  Note the anteriormost appendages, a pair of small pincers termed chelicerae.  These structures are also found on scorpions and the extinct eurypterids,  and are the food-gathering apparatus of these animals.
   I collected this specimen along the north shore of Long Island in 1999.  Note the slit (opening) at the very front of this animal.  The specimen is a
MOLT, and the animal crawled out ot its shell at this point in order to grow to a larger size. If fossilized, this specimen would probably be interpreted to be the actual animal. 
   Within the eurypterid-bearing Salina and Bertie Groups of New York (and adjacent area), I believe that all of the eurypterid specimens recovered are molts (when they appear whole) and the rest of the specimens (the most common remains) are disarticulated animals exhibiting every degree of preservation (from single tergites and telsons, to various attached tergites and carapaces, to swimming legs and individual joints--eg. metastoma, coxa, and individual spines of the walking legs).
Euproops danae
White areas are mineralizations.
Euproops danae
Nodules, like the Mazon Creek nodules shown above, are common in the Pennsylvanian deposits of Indiana and Illinois. While plant remains are abundant in the nodules, eg. ferns, many rare animals have been discovered over the years.
CIURCA 012401-1
Mesolimulus walchi Demarest (1822)
Solnhofen Lithographic Limestone
Solnhofen, Bavaria, Germany
Below, another example.