378 GEOGRAPHICAL AND GEOLOGICAL DISTKIBUTION. 



whether there is even u single species from the Cambrian which 

 also forms part of the Silurian fauna. There are apparently but 

 three forms — Calymene Blumenbachii, Dalmanites caudatus, Sphs:- 

 rexochus mirus— which in England connect the Lower and Upper 

 Silurian faunas, and an equally small number which unite the 

 Silurian with the Devonian. In Bohemia the number of Lower 

 and Upper Silurian connecting- forms is somewhat larger; but even 

 here the proportion to the entire fauna — nine out of three hundred 

 and twenty-three — is very small. The limitation in most cases is 

 even more pronounced, fixing the species to definite horizons of the 

 broader formations. 



With reapect to horizontal specific distribution, instances of broad 

 dispersion are not exactly uncommon. A fair proportion of the 

 Bohemian species, for example, are spread throughout Sweden, 

 Italy, Russia, and England, and a number have also been indicated 

 as occurring in the equivalent deposits of the North American con- 

 tinent. Calymene Blumenbachii is the commonest form occurring 

 on both sides of the Atlantic, in both regions being alike a Silurian 

 and Devonian species. 



Coincidently with the decline of the Trilobita we note the ap- 

 pearance of crustacean forms (Eurypterida) which hold a some- 

 what intermediate position between these and the modern king-crab 

 (Limulus), whose remains are first met with in rocks of Jurassic 

 age. Of this singular order, whiph comprises the giants of the 

 class, some half-dozen or more genera are recognised, whose com- 

 bined range includes the Upper Silurian and Carboniferous deposits. 

 In the Ludlow (U. Silurian) rocks of Britain there are no less than 

 thirty-two species of this order, representing six genera — Eurypter- 

 us, Pterygotus, Himantopterus, Slimonia, Stylonurus, and Hemi- 

 aspis, the last a transitional type connecting the group with the 

 Carboniferous limuloid forms constituting the family Bellinuridee 

 (Bellinurus, Prestwichia, Euproops). The most ponderous indi- 

 viduals of the order (Pterygotus Anglicus, Slimonia^coticus), which 

 are at the same time the largest of all known Crustacea, recent or 

 fossil, measm'ing from five to six feet in length, do not appear 

 until the Devonian period, or not until the group had attained to 

 considerable development. The simultaneous appearance among 

 the trilobites of the largest and smallest forms — Paradoxides, 

 Agnostus — would seem to point to a contrary order of develop 



