oKIMJK III 



PROPARIA 







pointed, ;ui(l usually with more -egments than in i'lun-nj,.*. ( >rdo\ jrian and Silurian; 

 Europe. 



Daknanites, Kmmri.h Figa L261, m7. /'. '-. I :',:'.<. I :n i . <;],iL.n ;l with 



well<- marked lateral furrow-: genal angle- produced into 

 spines; eyes large, prominent, ami with many di-tinct facet-; 



pygidium triangular, frequently pointed or niacromtad, with 

 more than eleven segment.-, -.met inies twenty <! more. < )i-do- 

 vician to Devonian ; Kuiope, America, and Ka.-t India. 



Chasmops, M'Coy (Fig. i:*17,//. Ditfer- from I inl,,,,/ ,,,'t,. ^ 

 in having the second pair of glabellar loin- nearly -epa rated 

 from the axis, in the obeoleeoenoe <>f the third pair of lol-, 

 and in its rounded pygidium. Ordovieian ; Kurope. 



Vertical Range and Distribution of Trilobites. 



Trilobites are the only large division of the Arthr]><l" 

 which has become extinct. K\vn in the earlie-t ('aml.rian 

 they bear evidence of great antiquity, in their diver.-itied 

 form, larval modifications, polymerous head, and caudal shield.-. 

 These features show that Trilobite phylogeny must extend far 

 liack into pre-Cambrian times. 



The maximum development of Trilobites occurred in the 

 Cambrian and Ordovician, after which they steadily waned 



both in numbers and variety. The genera of the Conocory- /;,,/,,, /,,. N //, /,-,. 

 vhidae are wholly restricted to the Cambrian, and here also Green. Silurian : Lockport] 



e i i 11 ^.i r\i j j t i-j i L New York (after Hall). 



are found nearly all the Olenidae and Aynostidae, only scattering 



representatives of which survive into the Ordovician. The Asaphidae are the >ole 

 remaining family found in the Cambrian, and they are more characteristic of the 

 Ordovician and Silurian. 



All families of Trilobites are present in the Ordovician, and continue into the 

 Silurian, with the exception of the Conocoryphidae, Aiiin>.<f/il>i>, and "/- /r />//., which 

 are properly Cambrian types. The 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 Proetidm; are met with in the < 'arboniferoiis. 

 and the single genus Phillipsia alone persists a* late as the Permian. 



As regards their geographical distribution, some genera are of cosmopolitan occur- 

 rence : such as Agnostus, Conocoryplu>, I'fi/r}i<i/i'i ///, 7'" /<></',., /'/.-, 7V//M//7. //>-, 

 Illaenus, Proetus, Phillipsia, Addas-pis, Lichas, Calymmene, Homalonotus, 

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

 limited in distribution, so that a laige number of genera found in Sweden, Bohemia, 

 England, and North America are unknown outside of certain very rot ii. ted areas; 

 and the total number of species common to both sides of the Atlantic is very .-mall. 



A remarkable contrast is observable between the older Palaeo/oic Trilobites of tin- 

 northern parts of Europe, and those of the middle and southern portion-. While the 

 majority of northern genera and species are common to Great Britain, Scandinavia, 

 and Russia, the forms of the central Kuropean provinces (Bohemia, Thuringia, Fi<-htel- 

 berg, the Hartz, Belgium, Brittany, Northern Spain, Portugal, the Pyreiiee.-, the Alp-. 

 and Sardinia) are so dir-similar as to stand in closer relationships with the North 

 American than with the first-named Trilobite fauna. Of the :}.">' >perie> found in 

 Norway and Sweden, and of the 275 in Bohemia, only >ix are common to both 

 provinces, and it is doubtful if the.-e are really identical. 



The first of the accompanying tables shows tin- range and relative de\e]opm-nt of 

 the orders and the class; the second represents the vertical range of tin- 

 families of Trilobites. 



