264 



NORTH AMERICAN INDEX FOSSILS. 



22. M. vermontana Hall. (Fig. 1555,/.) , Cambric. 

 Thorax of 26 segments. (Type of genus.) 



Georgian of Vermont, Labrador and Newfoundland. 



23. M. asaphoides (Emmons). (Fig. 1556, &-c/.) Cambric. 

 Third thoracic segment only occasionally longer than the rest; 



segments 18, the anterior 13 larger than the posterior 5 ; on each of 



Fig. 1556. a, Holmia broggeri, X /^ ; b-d, Mesonaces asaphoides : b, adult, X % V 

 c, pygidium, and last thoracic segment without the spines, X /^ 5 ^> young cephalon, 



XVz- (After Walcott.) 



the 5 smaller posterior segments is a long spine projecting back 

 over the transverse, plate-like pygidium. Form broader than in 

 preceding. 



Georgian of New York; Etcheminian of Massachusetts ( ?). 



XIII. Ellipsocephalus Zenker. 



Cephalon semicircular, depressed, without spines. Glabella 

 smooth, obtusely angular in front. Free cheeks short, narrow; 

 thoracic segments 12-14; axis nearly as broad as 'lateral lobes. 

 Pygidium small, semicircular. Lower and Middle Cambric. 



