146 GEOLOGICAL DISTEIBUTIOK. 



estimated at between two and three thousand. The genus Gonia- 

 tites, which effects a partial transition between the Nautilidse and 

 the Mesozoic ammonites, appears for the first time in the later 

 Silurian deposits. 



It is a surprising fact, considering the remarkable development 

 of the Cambrian trilobitic fauna, that not only are none of the 

 earlier species represented in the Silurian deposits, but that by far 

 the greater number of generic tjrpes, and more particularly those 

 which by a special individual or specific development are rendered 

 most important, as Paradoxides, Dikelocephalus, Olenus, Sao, and 

 Conocephalus, should be also wanting. Only seven out of some 

 twenty-seven genera of the primordial zone connect the formations of 

 the two periods. The Silurian Trilobita comprise probably in the 

 neighbourhood of fifteen hundred species, referable to some fifty or 

 more genera ; yet of this vast number there are barely a half-dozen 

 species which transgress the boundaries of the formation, passing 

 into the Devonian. Among the more abundantly represented gen- 

 era are Phacops, Dalmania, Calymene, Asaphus, Trinucleus, Aci- 

 daspis, and Cheirurus. 



Viewed irrespective of numerical development, the most signi- 

 ficant feature connected with the Silurian invertebrate fauna is the 

 introduction of the earliest "air-breathers." Until recently these 

 were supposed to belong to the period following, the Devonian, but 

 the discovery of a true scorpioid (Palseophoneus) in the Upper Silu- 

 rian deposits of both Sweden and Scotland, and of an apparent 

 orthopteroid (Palseoblattina) in the nearly equivalent deposits of 

 Calvados, France, proves conclusively that a very considerable 

 difEerentiation among the air-breathing arthropods had already 

 taken place, and points to a period very much more ancient for the 

 first origination of the group.* 



Devonian Fauna. — The primitive air-breathing arthropod fauna 

 just referred to finds a somewhat larger extension in the rocks of 

 Devonian age, where fragments belonging to some five or six spe- 

 cies of insect, possibly representing as many genera — Platephemera, 

 Gerephemera, Lithentomum, Homothetis, Xenoneura, Discrytus — 

 have been discovered. These appear to belong to the modern 



* The scorpion described by Professor Whitfield (" Science," July 31, 

 1885) from the Upper Silurian rocks of New Yorls: may, as suggested by Mr. 

 Pohlman (" Science," September 4, 1885), prove to be a young eurypteroid. 



