ARTHROPOD A — CRUSTACEA — TRILOBITES 



297 



in comparison to its length, with long genal spines (borne by the 

 free cheeks) and a regularly pitted border, whence its common 

 name, — lace-collar trilobite. The glabella 

 is prominent, with a strongly convex area 

 on each side, giving thus three prominent 

 elevations to the cephalon (whence the 

 name from Latin tres (tri-), three, + nu- 

 cleus, little nut). Eyes are usually absent ; 

 the thorax has six segments ; the axial lobe 

 is narrow. 



It is abundant in North America and 

 Europe. 



Fig. i2g. — Trinucleus 

 conceniricus Eaton. 

 Natural size. The 

 lace-collar trilobite, 

 abundant in the Or- 

 dovician seas of 

 North America and 

 Europe. (After Lo- 

 gan.) 



1. Sketch specimen, labeling the portions 

 preserved. 



2. What is the significance of the common name? The 

 technical name ? 



3. What part of the living animal do fossils of trilobites repre- 

 sent? 



Calymene (Fig. 130). 



Fig. 1,50. — Calymene 

 niagarensis Hall. .\ 

 trilobite living in the 

 seas of the Silurian 

 period. Dorsal view. 

 Natural size. (After 

 Hall.) 



Ordovician- Devonian. 



Cephalon with a thickened margin, the 

 facial suture cutting the lateral margin 

 immediately anterior to the genal angle; 

 the glabella is very convex, broadest be- 

 hind, divided by three pairs of lateral 

 grooves which define three rounded lateral 

 folds decreasing in size anteriorly ; the 

 eyes are small. The thorax is divided 

 into thirteen segments. Calymene is very 

 frequently found enrolled like the living 

 pill-bug, showing that it possessed well de- 

 veloped ventral muscles. By this means 

 it protected the more vulnerable ventral 

 surface of its body. 



Calymene is found in North America, 



