M B-CLAJM i Ti;ii.ni;iT.\ 



ti'li it'll until /.. Qf iinHnl in //"/</, n ml nii't'if tin '"//,/, n ml gSsHf 



eyes when present; ,-i-f>/i,ili<- appendage* //"///"////. <-/,/,.x/.x7,/,// /////,<. 



"// liii'iinioiix, and functioning ".> ambviUtiory <nl ">'"' /</" 



iliitnillilh'*, iHiii-ll an /'Hi-'!>/ * a air ;i. I'jijirr ///> Jofni liii/n in //-//- // 1,,],,,/ /,///- 



miller li/> /tri'fi-nf. >'//// /7r.< ,,j tin- /////!/.* /,/,/w,/, ///,// //, another, 

 ininiiii'r jri'iu ttrn in ttn utii-iiiii-. ji H /t,iiiii<iii tegtienU /-.//-/.//i/r /// ///////.. 



A; /"/'Mi (I Caudal *//////. /// .*;////'/>-, Ilinnn-ir a,,,/ /////A,////'/////, ////// " / 

 jnintnl lii-iiimmx limits. All /////As ///v //// /,./// ///////Vs /<///</// gnafk 



^ich become organs of manducation //// //// ///////. /,Vx/*//-////// ////"/>////<///<// ////// //// 

 branchial fringet on the exopodif'*. /ii-'ln/nnrnf j>r<><-<'n/iit<t //"/// " y//-/i/i/<// ///!///> 

 /O?T, the protaspis. A// the progressive <n/<//t/<u/ <>f *,/,, /</.< </f tuceesnn ///.////>. 



The Trilobites constitute a group of extinct marine animals, am! 

 related to the stock of the higher modern Ci'n*1<i<;,i they art- thcreforr to IP 

 considered as very primitive Crustaceans. The sub-class had its origin in JHL-- 

 Cambrian times. Trilobite remains are very abundant in the oldest known 

 fossiliferous strata, the Cambrian, where they exceed in number and div-i>itv 

 all other forms of animal life. They continue to be very plentiful in tin- 

 Ordovician and Silurian, but decline in the Devonian, and the few last sur- 

 vivors are found in the Carboniferous and Permian. Probably there havr 

 been more than two thousand species described, distributed among nearly t\\<> 

 hundred genera. These numbers give an idea of the amount of differentiation 

 and specialisation attained by Trilobites during Palaeozoic times. 



Carapace. Trilobites were covered or protected on the dorsal side by a 

 hard crust or shield, which is the only portion commonly preserved. Their 

 remains are therefore readily recognisable by the form and structure of this 

 shield. It is divided longitudinally by two dorsal furrows, or grooves, into 

 three portions or regions, and on this account the name Trilobite was first 

 given. The central part forms the axis of the animal, and contained tin- 

 principal organs, as the viscera, heart, and chain of ganglia. Transversely tin- 

 shield is divided into (a) a head portion called the cephalon ; (li) a series of 

 joints or segments, forming the thorax; and (c) a tail-piece or pygidium, 

 forming the abdomen. 



The test seldom exceeds one millimetre in thickness, and consists of thin 

 laminae of carbonaceous and phosphatic compounds of calcium, some of which 

 were originally chitinous substances. The laminae are frequently traversed 

 by minute pores, which give a punctate appearance to the test, and are some- 

 times large, as in H(ninilt>n<>fiis and related forms. 



The carapace is somewhat arched or convex, generally elongate-oval in 

 form, and rounded at both ends. The length is almost invariably greater 

 than the width. Very often the same species shows a broad form. M \\< !! 

 as a relatively larger, nan-own- one. The former was considered l>y 

 Barrande as representing the female, and the latter the male individual. Tin- 

 carapace is often ornamented with spines, teeth, and knobs. These may be of 

 the nature of surface ornaments, or in the case of spines, may be produced by 

 growths from the genal angles, the ends of the segments of the thorax and 

 pygidium, or the spiniform extension of the pygidial termination. 



The carapace does not often terminate at the margin as a simple lamellar 



plate, but is turned under, and forms a reflexed margin, or i/nuHm-i, which i- 



parallel to the outer edge, but is separated from the upper surface by a narrow . 



partially included space. This produces the hollow spines from the ends of 



VOL. 1 2 B 



