258 



NORTH AMERICAN INDEX FOSSILS. 



except the occipital one, though at times there are two others 

 well defined. Pygidium well segmented. 

 Lower Cambric of New York. 



7. M. pulchellus Hartt. {M. punctatus of American authors.) 

 ( Fig. 1 543, h, i. ) Cambric. 

 Cephalon with crenulated border and basal spine. 



Middle Cambric of St. John group of New Brunswick and New- 

 foundland. 



III. Harpes Goldfuss. 



Cephalon large, with a broad marginal expansion. Glabella short 

 and prominent. Free cheeks ventral. Facial sutures marginal. 

 Eye-spots paired and simple, and on the fixed, not the free, cheeks. 

 Thorax of 25 to 29 segments ; pleura long and grooved. Pygidium 

 very small, of three or four segments. Ordovicic and Siluric. 



8. H. (Harpina) ottawaensis Billings. (Fig. 1546.) Ordovicic. 

 Marginal expansion of cephalon strongly punctate. 



Trenton of New York, New Jersey, Minnesota, Quebec, etc.; 

 Chazy of New York, etc. 



IV. Trinucleus Lhwyd. 

 Cephalon very broad proportionately, with long genal spines 

 and broad, regularly pitted border. Glabella inflated, pear-shaped, 



Fig. 1546. Harpes ottatvaensis, 

 (After Billings.) 



Fig. 1547. Trinucleus concentricus. a, 

 cephalon showing occipital spine ; b, entire in- 

 dividual, but without the median spine. (After 

 Logan. ) 



smooth or with indistinct furrows. Eyes generally absent. Thorax 

 of six segments which are nearly straight at their extremities ; axis 

 narrow. Pygidium with margin entire. Ordovicic. 



