82 



Guide to AracJiuida. 



Table-case exceed ten in number. The American King-cral) [Xiplwsura 

 w°'ii^^" l^oJyphcinus) differs from tlie Oriental species in having the 

 No. 7. terminal segment of the inner branch of the genital operculum (on 

 each side) retained as a free movable lobe, whilst in the Oriental 

 genera {Tachyplcus and Carcinoscorpius) it is suppressed. The 

 three genera which have resulted from the subdivision of the old 

 genus Limidus are referable to a single family, Xiphosuridac. 



The King-crabs are marine, shore-frequenting forms. They live 

 in water of moderate depth, burrowing in the sand at the bottom, 

 and their food consists of l)ivalves, worms, etc. They occur on 

 the Eastern coast of North and Central America, and in the 



Fig. 54. 



The American King-crab (Xiphoaura polijpUemus). 

 About tV the diameter of the animal. 



Oriental seas from the Bay of Bengal to the coasts of China and 

 Japan, Torres Straits, etc. A number of small Palaeozoic 

 forms (e.g. Belinurus and Hcmiaspis, of which figures are shown 

 in Table-case 18) are known, which seem to l)e intermediate in 

 structure between the Xiphosura and the Trilobites. Forms 

 w^hich resemble the modern type of Xiphosura first appear in the 

 Triassic rocks. Several specimens of King-crabs from the 

 Solenhofen stone (Jurassic Period) are shown in the Geological 

 Department. (Gallery 8, Wall-case 13c.) 



A large example of a King-crab (TacJajplcus iridoildiiis) from 

 Britisli North Borneo is displayed in the upper part of Wall-case 7, 

 and representatives of the three genera are shown in Table-case 18. 



