254 Annals of the Carnegie Museum. 



Black River Limestone at the falls of La Petite Chaudiere. One speci- 

 men shows the larger part of a free cheek. In this specimen the genal 

 angle is rounded, not drawn out into a spine as in the specimens from 

 the Chazy referred to this species by Raymond. The dorsal furrows 

 on the specimens from the Chazy are also much straighter than those 

 of the specimens from the Black River. It seems very possible that 

 the form from Valcour Island represents another, but closely allied, 

 species. 



A specimen from Mechanicsville shows fragments of four thoracic 

 segments, similar in size and shape to those of Bumastus billingsi. 

 Walcott says of the thorax of his specimens: " Thorax uniformly 

 arched, not trilobed ; nine segments only can be counted ; the crush- 

 ing together of the segments may have forced one beneath the head." 

 From the shape of the thorax as shown by these two specimens, it 

 seems probable that this species should be referred to Bumastus. 



Locality. — This species is found rather rarely in the Black River 

 Limestone at Mechanicsville, Ontario, and Tetreauville, Quebec. 



Summary. 

 In the preceding pages, one new species of Illcenus from the Black 

 River, and two new species of Bumastus from the Trenton Limestone 

 have been described. The thorax and pygidium of Illcenus angusticollis , 

 previously unknown, have been described, and the differences between 

 Thaleops ovata and Thaleops arclura pointed out. Reasons are given 

 for retaining the specific name milleri for the small Bumastus described 

 by Billings, and Bumastus indeterminatus is figured for the first time 

 from specimens obtained in the Black River formation. 



EXPLANATION OF PLATES. 

 Plate LX. 

 i. Illcenus americanus Billings. Thorax and pygidium of a specimen from the 

 Trenton Limestone, Hull, Canada. One third larger than natural size. 



2. The same species. A cranidium, viewed from the front. Same magnification 

 as the preceding. 



3. The same species. The same pygidium as shown in Fig. 1, looked at from 

 above. One third larger than natural size. Compare Fig. I with 4, and 3 with 6 

 and 7. 



4. Illcenus latiaxiatus Narraway and Raymond. An imperfect thorax and 

 pygidium. One third larger than natural size. 



5. The same species. A pygidium viewed from the side. 



