MWWSfl 



146 



INVI'.IiTF.liUATK ANIMALS. 



C)|:DER TuiLOlHTA. 



Tile Triloliites cniistitute a wlinlly extinct order of tlie Cnistaeea, 

 anil deserve a Hliort notice from their great geological importance. 

 Thev deri\ e their name from the fact that the body e.\.liibits a more 

 or li'ss conspienons diviHion into .a central and two lateral lobes 

 (tig. !•■")). The entire .shell or (a-nst is conijiosed of an anterior .senii- 

 circul.-i,r shielil, covering the heail, a series 

 of mov.able rings con.stituting tlie thorax, 

 anil a tailpiece composed of amalgamated 

 segments, and I'epresenting the abdomen. 

 ( >n the miller surface of the shell nothing 

 S^\ had niitil Lately ever been discovered ex- 

 J(\ cejit the njijier lip, but recently limbs and 

 br.uichia' have been made out. The cephalic 

 shield usually bears a ]i.'iir of compound 

 eyes, but tliesi' are si uiietimes wanting. It 

 i-i known tli.at most of the Trilobites pos- 

 sessed the ])ower of rolling themselves up 

 into a. l.iall, much as our modern wood-lice. 

 The Trilobites are only known as occurring 

 in the older rocks of the earth's crust, .and they .are cliieHy character- 

 istic of the period known to geologists as tlie " ,Silurian." 



(Attrr S.ilti']-.) 



0::dk.u Piiyi.loi'oh.v. 



This is a sm.all order coni]irising a number of ( 'rnstaceans, which 

 are )iriiiei]i,illy foniid in iiiliind waters, whether fresh or salt, only 

 one form inh.abiting the .sea. These ( 'rustace;ins (lig. ;)(>) have 



iinmeroiis feet, never less than sixteen in number, and the feet are 

 iiiiislly expanded .and le.'if-like, and serve a.s gills or bre.alhing-org.an.s. 

 'I III' III Ills' nriy be u.'ikrd, or the he:id and thorax may be coyered by 



