Remarks.-The Xiphosura are marine in habit, frequenting the shore. They are represented at the present day by the single genus Limulus (figs. 44 and 45; also figs. 7, 9, 11, to 15 and 20), often termed the king-crab, which occurs on the American coast of the Atlantic Ocean, but not on its eastern coasts, and on the Asiatic coast of the Pacific. The Atlantic species (L. polyphemus) is common on the coasts of the United States, and is known as the king-crab or horse-shoe crab. A single specimen was found in the harbour of Copenhagen in the 18th century, having presumably been carried over by a ship to which it clung.
A species of Limulus is found in the Buntersandstein of the Vosges; L. Walchi is abundant in the Oolitic lithographic slates of Bavaria.
 | Fig. 45.—Ventralview of Limulus polyphemus. |
1 to 6, The six prosomaticpairs of appendages. abd, the solid opisthosomaticcarapace. tels, the post-anal spine(not the telson as thelettering would seemto imply, but onlyits post-anal portion). operc, the fused firstpair of mesosomaticappendages formingthe genital operculum. | |
| (From Parker and Haswell,Text book of Zoology, afterLeuckart.) |
The genera Belinurus, Aglaspis, Prestwichia, Hemiaspis and Bunodes consist of small forms which occur in Palaeozoic rocks. In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitae. The young form of Limulus itself (fig. 40) is also similar to a Trilobite so far as its segmentation and trilobation are concerned. The lateral eyes of Limulus appear to be identical in structure and position with those of certain Trilobitae.
 | Fig. 46.—EurypterusFischeri,Eichwald. Silurianof Rootzikil.Restoration afterSchmidt. Thedorsal aspect ispresented showingthe prosomaticshield with pairedcompound eyesand the prosomaticappendagesII. to VI. Thesmall first pair ofappendages is concealedfrom viewby the carapace,1 to 12 are thesomites of theopisthosoma; 13,the post-analspine. |
| (From Zittel’s Text-bookof Palaeontology,The Macmillan Co,New York, 1896.) |
Order 2. Gigantostraca (figs. 46, 47).—Free-swimming forms, with the appendages of the 6th or 5th and 6th pairs flattened or lengthened to act as oars; segments of mesosoma and metasoma (= opisthosoma), twelve in number.
Appendages of anterior pair very large and chelate.
| Sub-order Pterygotomorpha, Pterygotidae (Pterygotus). |
Appendages of anterior pair minute and chelate.