TRILOBITA—OPISTHOPARIA. 263 



like spine. Surfaceof test covered with inosculating strise. Lower 

 Cambric. 



18. 0. thompsoni Hall. (Fig. 1555,0, &.) Cambric. 

 Large, very slightly convex; spines of third thoracic segment of 



moderate length. (Type of genus.) 



Georgian of Vermont, New Jersey, Pennsylvania, Quebec, New- 

 foundland, Labrador, etc. 



19. 0. gilberti Meek. (Fig. 1555, c-^.) Cambric. 

 This western species is very similar to O. thompsoni,hut it shows 



great variations in different individuals in width of body, length 

 of genal and third pleural spines, position of genal spines, size of 

 eye lobes, etc. It is also moderately convex. 



Lower Cambric (Georgian) of Utah, Nevada and British 

 Columbia. 



20. 0. iddingsi Walcott. (Fig. 1552, d.) Cambric. 

 Outline of head subtriangular. 



Lower Cambric (Georgian) of Nevada, etc. 



XL HoLMiA Matthew. 



Cephalon semicircular ; glabella with nearly parallel sides ; a very 

 long occipital spine present. A small secondary spine occurs on 

 each side, just within the genal angle. Thorax of 18 segments, 

 with a short spine upon each segment of the axis; pleura broad 

 almost to the tip. Pygidium small, subquadrangular. 



Lower Cambric of Atlantic provinces of America and Europe. 



21. H. broggeri Walcott. (Figs. 1556, a; 1556,^.) Cambric. 

 Posterior border of cephalon cut by a notch just within the 



genal spine. 



Lower Cambric (Etcheminian) of Massachusetts and New- 

 foundland. 



XIL Mesonaces Walcott. 



Differs from Olenellus in that the anterior segments of the 

 thorax are larger than the posterior, and in the plate-Hke pygidium. 

 Anteriorly like Olenellus; posteriorly like Paradoxides. Lower 

 Cambric. 



