in 



PROPARIA 



,iti;il,'.< 



lateral 



apinote, ''/</ /<// 



nfni. lilitlhlln //'//// //- 



<7i<r/-x //</-, x.yi/f /.//.. 



Family 7. Acidaspidae. 



lijitn-nl, iiiinili-"- 



,,,,,lni,i ,11 nil /.,/,. ,,,/ /,--, Of 

 - ,/, /,.///,./ 



forward^ 



cutting fli>- anterior //"/.//// "// .>// i>f tin //<//.-///. 



segments, u- it h //</</</ y//. //// .-/, ,,,/,r/ /////< ImUmr .< { . 

 Pygidium usually .>///>//, i/v'/A .</<///n//x ///(//(////. Oi-il-.- 

 vician to Devonian. 



In this family, as well as in the Li<-lnnliil>i> , \- to 

 be found tin- high-st expression of differentiation and 

 specialisation among the Opt0<A0pana, The primitive 

 peutamerous lobation of the axis of the cianidium is 

 entirely obscured, and is only clearly seen in the protaspis 

 and early nepionic stages. These two families are very 

 closely related, the chief differences being noted in tin- 

 size and character of the pygidium, and the ribbed or 

 grooved pleura. The Lichades are generally much 

 larger and flatter, but the smaller and spinose forms of 

 Arges and Ceratolichas approach quite near some of the 

 Acidaspidae. 



Acidaspis, Murch. (Figs. 1276, 1288, /, 1316). 

 Ordovician to Devonian ; Europe and North America. 



Dicranurus, Conrad; Ancyropyye, Clarke; Devonian. (Et. E); St 

 Selenopeltis, Corda ; Ordovician. 



FIG. 1316. 



A<-i'l"/i!.-< li>n',; ,.,.:_ Ban. Silurian 



I : ; 



Order 3. PROPARIA. Beecher. 



Free cheeks not bearing the genal angles. Facial ntfwrei extending from tin- 

 margins of the ceplialon in front of the genal /<///-.>, iim-m-il mul ///-, /w/,/, cutting th- 

 anterior margin separately or uniting in front of the glabella. Compound paired 0yf 

 scarcely developed or sometimes absent in the most primitive family ; "'<H <I> r,l<,j>.<f ,///// 

 schizochroal in the highest family. 



This is the only order of Trilobites which apparently lu-gins during the known 

 Palaeozoic, and unlike the other orders, had no pre-< 'ambrian <-xi~t-m -. The earli-.-t 

 forms of the Proparia were initiated at the close of the Cambrian and dawn of the 

 Ordovician. The greatest generic differentiation of the group was early attained : 

 during the Silurian and Devonian a rapid decline niMied, and only .n- <T two genera 

 survived into the beginning of the ( Sarboniferoua 



Among the Opifthoparia, it was shown that the Conocoryphidat formed tin- 

 natural base or most primitive family in the order, and was di>tingui>hed by tin- 

 narrow marginal free cheeks and absence of well -developed eyes, It i.- <-t 

 interest and importance to be able to recognise in tin- l'r"/i>irin a Hinilar primitive 

 family having characters in common \\iih the other, but still clearly belonging 

 to the higher order. Finn, j,,, ,/', Arn'n, and Dindymem <>t' tin- /;//.;////., 

 constitute a group of apparently blind Trilo1>ites with narrow marginal tVee cheeks, 

 and present in general the appearance of At/>.<. i'nn<-<>njtihe, Ctenocq)lil"<. etc., of the 

 Conocoryphidae. 



