SUBCLASS I TRILOBITA '727 



Subfamily C. Pterygometopinae Reed. 



Rather small Phacopidae, the cephalon with more or less modified pentamerous 

 lobation, and the pygidiuvi less triangular and with fewer seg- 

 ments than in most of the Dalmanitinae. 



Pterygometopus Schmidt (Fig. 1412). Glabella with three 

 pairs of lateral furrows. Ordovician ; Europe and North 

 America. 



Ghasmops M'Coy (Fig. 1398, H). Second pair of glabellar 

 lobes absent, or represented by tubercles. Ordovician; Europe. (j)l\m!)T'^'^ oi\\oy\cf^n\ 



Monoraicos Schmidt. Second and third pairs of glabellar iswos, Esthonia (after 

 furrows represented by pits. Ordovician ; Europe. 



Fir,. 1412. 



Schmidt). 



Geological Range and Distribution of Trilobites. 



Trilobites are the only large division of the Arthropoda which has become extinct. 

 Even in the earliest Cambrian they bear evidence of great antiquity, — in their 

 diversified form, larval modifications, polymerous head and caudal shields. These 

 features show that Trilobite phylogeny must extend far back into pre-Cambrian 

 times, and it is probable that primitive Branchiopods, of a type corresponding to the 

 modern Apus, were developed even earlier. The views of Bernard and Walcott 

 regarding the origin of Trilobites and higher Crustacea from a primitive Apus-lika 

 ancestral stock are mentioned a few pages farther on under the head of Branchiopoda. 



Concerning the habits of Cambrian Trilobites Dr. Walcott has suggested that the 

 adult animals probably crawled about the sea-bottom and did not swim freely in the 

 water to the extent that it would be necessary to see the bottom. Their habits must 

 have been very much like those of Limulus when in search of food. That the 

 creatures burrowed and pushed their way through the mud and soft sands is proven 

 by the trails and burrows made by them, some of which we now designate as 

 Protichnites. 



The maximum development of Trilobites occurred in the Cambrian and Ordovician, 

 after which they steadily waned both in niunbers and variety. The genera of the 

 Conocoryphidae, Eodiscidae, Mesonacidae, Paradoxidae, Oryctocephalidae and Ellipso- 

 cephalidae, are wholly restricted to the Cambrian, and here also are found nearly all 

 the Olenidae and Agnostidae, only scattering representatives of which survive into 

 the Ordovician. The Asaphidae are more characteristic of the Ordovician, and the 

 Cryptolithidae, Shumardiidae, Remopleuridae, Bathyuridae and Aeglinidae are 

 restricted to it. The Raphiophoridae, Goldiidae, Harpedidae, Encrinuridae, and 

 Illaenidae flourished in the Ordovician and Silurian, while the Proetidae, Lichadidae, 

 Odontopleuridae and Phacopidae attained their greatest development in the Silurian 

 and Lower and Middle Devonian. 



The later Devonian witnesses a decline in the number of families present, and with 

 the close of this era, the class practically became extinct, since only five genera of 

 one family, the Proetidae, are met with in the Carboniferous, and the single genus 

 Phillipsia alone persists as late as the Permian. 



As regards their geograj^hical distribution, some genera are of cosmopolitan occur- 

 rence : such as Agnostus, Gonocoryphe, Ptychoparia, Paradoxides, Gryptolithus, lUaenus, 

 Proetus, Phillipsia, Acidaspis, Lichas, Galymene, Homalonotus, Gheirurus, Phacops, 

 Dalmanites, and others. The majority of forms, however, are extremely limited in 

 distribution, so that a large number of genera found in Sweden, Bohemia, England 

 and North America are unknown outside of certain very restricted areas ; and the 

 total number of species common to both sides of the Atlantic is very small. 



A remarkable contrast is observable between the older Paleozoic Trilobites of the 



