82 
Guide to Arachnida. 
Table-case exceed ten in number. The American King-crab ( Xiphosura 
Wa'l/case P°typhemus) differs from the 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 ( Tachypleus and Garcinoscorpius ) it is suppressed. The 
three genera which have resulted from the subdivision of the old 
genus Limulus are referable to a single family, Xiphosuridae. 
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 bivalves, worms, etc. They occur on 
, the Eastern coast of North and Central America, and in the 
Fig. 54. 
The American King-crab ( Xiphosura joolyphemus) . 
About 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 Hemiaspis, of which figures are shown 
in Table-case 18) are known, which seem to be intermediate in 
structure between the Xiphosura and the Trilobites. Forms 
which 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 ( Tachypleus tridentatus) from 
British North Borneo is displayed in the upper part of Wall-case 7, 
and representatives of the three genera are shown in Table-case 18. 
