312 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The best known example is the horseshoe crab, — Limulus 

 Polyphemus (Fig. 124). In this the six pairs of appendages are 

 all chelate except the sixth, which is surrounded with whorls of 

 plates which aid in pushing the animal over or through the mud. 

 Of the seven abdominal segments the six anterior ones consist 

 of operculum and five pairs of book-gills ; there are over one 

 hundred pairs of gill leaves in all. This species lives in shallow 

 water, grubbing in the mud for its food. It ranges from Yucatan 

 to Maine. The eggs hatch in July and August. The young are 

 tri-lobed like the trilobites ; they swim much upon their backs. 

 This is the last surviving genus of an order whose members were 

 abundant both in number of genera and species throughout the 

 world from early Cambrian times. (See relationship of the 

 Arachnida, p. 309.) The only other living species (L. mol- 

 luccaniis) occurs off the east coast of Asia and in the Malay 

 Archipelago. Limulus occurs from the Triassic to the present. 

 The many other representatives of this order exist only as fossils, 

 as Belinurus (Devonian to Pennsylvanian), Neolimulus (Silu- 

 rian) and Aglaspis (Cambrian). 



1. Sketch the horseshoe crab, side view, showing one half 

 of the dorsal carapace. Label carapace, compound eye, telson, 

 book-gills, operculum, legs. 



2. Where does Limulus live? How? 



3. Why is it placed under the Arachnida, and not under the 

 Crustacea ? 



4. What is its geologic range ? 



5. Why do we expect an order with but a single living genus 

 to have had a long ancestral history, with many fossil represen- 

 tatives ? Mention examples of this among plants. 



Order 2, Eurypterida 



Body elongate with comparatively small cephalothorax and 

 an abdomen of thirteen segments, including the post-anal tel- 

 son. There are two large, lateral, faceted eyes,~an3r a pair 

 of median, simple ones. Six pairs of legs surround the mouth, 



