Trilop.itf.s or ihf. Chazv Limestone. 365 



7'Ii)rax. — There are nineteen thoracic segments. The axis occu- 

 pies about one third the width of the thorax and tapers very gradually 

 toward the pygidium. The trilobite is widest at the posterior angles 

 of the cephalon, but tapers only a little posteriorly. The ]:)ygidium 

 forms almost a semicircle. On the axis the segments bend forward 

 very considerably, while on the side lobes their course is almost 

 perpendicular to the axis ; at the sides they turn backward and curve 

 sharply downward. 



Pygidium. — The pygidium is nearly semicircular, convex, steep at 

 the sides and back. Axis of medium width, with six annulations, the 

 last triangular. There are five ribs on each side, extending, as spines, 

 beyond the margin. These spinose terminations are quite close together, 

 the spaces being of less width than the spines. 



Measuroiieiits. — Largest specimen : length 78 mm.; width back of 

 cephalon, 40 mm.; length of cephalon, 18 mm. Eyes 25 mm. apart ; 

 5 mm. from back of cephalon ; 4 mm. from glabella. Thorax 48 mm. 

 long. Axis 14 mm. wide at cephalon ; 1 1 mm. at pygidium. Thorax 

 29 mm. wide at pygidium. 



Second in size: length 38 mm.; width back of cephalon 21 mm. 

 Cephalon 9 mm. long. Thorax 25 mm. long. Axis 8 mm. wide at 

 front ; 5 mm. at pygidium. Thorax 15 mm. wide at pygidium. 

 Distance between eyes 13 mm. They are situated 2 mm. from the 

 glabella and 2.5 mm. from the posterior margin. 



A small specimen: length 31 mm.; width back of cephalon 18 

 mm. Eyes 12 mm. apart; 2 mm. from glabella; 1.5 mm. from the 

 posterior edge. Thprax 18.5 mm. long. Axis 6. 5 mm. wide at ceph- 

 alon ; 4 mm. wide at pygidium. Length of cephalon, 7 mm. Width 

 at front of pygidium, 6 mm. 



Locality. — Valcour, Valcour Island, Chazy and Cooperville, New 

 York : Isle La Motte, Vermont ; Montreal and Mingan Islands, 

 Canada. The pygidium figured is from a specimen in the Carnegie 

 Museum. 



Genus CERAUR US Green (sensu stricto). 



Ceraurus pompilius Billings. (Plate 14, figure 14.) 



Cheiriirus roinpiliii:^ Billings, 1S65, Paleozoic Fossils of Canada, volume I, J^age 

 181, figure 162. 



The specimen on which this species was founded is so small a 

 fragment that it is with some doubt that we assign our specimens to 



