ARTHROPODA — CRUSTACEA — TRILOBITES 287 



other respects these genera are more specialized and hence less 

 like the trilobites than Apus. 



Triarthrus, a trilobite crustacean, lived in the marine waters 

 of eastern North America and northern Europe during the 

 Ordovician period. 



The body was entirely incased in a chitinous skeleton, har- 

 dened with more or less lime carbonate. Dorsally this skeleton 

 was thickened, forming the dorsal shield; it is this which is 

 usually preserved. It is divided into the head (or cephalon), 

 thorax and pygidium ; two furrows extend from the front of 

 the head to the end of the pygidium, separating a higher, narrow 

 median portion, the axial lobe, from a lower portion on each side, 

 the two pleural lobes. 



The portion of the axial lobe in the head is called the glabella ; 

 it is large, well-defined, subquadrate and marked usually by 

 two pairs of discontinuous furrow^s (glabellar furrows). (Com- 

 pare gla.f. in Figs. 126 and 127.) Extending from the poste- 

 rior angles of the head shield obliquely toward the glabella and 

 then anteriorly to the margin is the facial suture (compare 

 fa.s. in Figs. 126 and 128), dividing the sides of the head shield 

 into free Sind fixed cheeks, united by uncalcified chitinous joints. 



The thorax is divided by transverse furrows into fourteen to 

 sixteen segments, united by joints of chitin. 



The pygidium, which covers the abdomen, consists of a single 

 piece, though the superficial grooves divide it into axial and 

 pleural lobes. 



The head shield is anteriorly bent under and reflexed. To 

 this reflected border, or doublure, was attached medially a sepa- 

 rate plate, the upper lip, or hypostome. Posterior to this was 

 a small convex plate, the lower lip, or metastoyne. The animal 

 was further protected on the under side by a thin, uncalcified, 

 chitinous membrane (Fig. 126, B) divided into an axial and two 

 pleural portions corresponding to the axial and pleural lobes of 

 the dorsal shield. This ventral membrane was attached to the 

 upper and lower lips, to the reflected border of the head shield, 



