250 Annals of the Carnegie Museum. 



name, which is supported by a good description and a figure, by a 

 name which is dependent only on a rather poor figure. Moreover, 

 as we now know more than one species of Bumastns from the Black 

 River and Trenton formations, it is not at all certain that Emmons and 

 Billings described the same species. The specimens from Ottawa 

 invariably show nine segments, while Emmons' figure represents a 

 specimen with only eight. Clarke has described two specimens of 

 Bumastus from the Trenton at Trenton Falls, New York, one with 

 eight, and one with ten segments. The specimen with ten segments 

 differs from the specimens found in the Black River Limestone at 

 Ottawa in having a longer and narrower form, fainter dorsal furrows 

 on the cephalon and more delicate thoracic segments in addition to 

 having one more segment. In these smooth trilobites, all apparently 

 descended from the same stock, it is very difficult to seize upon dis- 

 tinguishing characters, even where the whole aspect of each animal is 

 quite characteristic. Where the form is constant, as in the specimens 

 at Ottawa, it seems important that there should be some ways of desig- 

 nating it. In this case it is only necessary to revive Billings' name 

 for the Bumastus at Ottawa, leaving to Bumastus trentonensis the 

 forms described by Clarke. 



This species has been too fully described by Billings to require a 

 formal description here. It is very abundant in the Black River 

 Limestone about Ottawa, and though the collection contains speci- 

 mens varying in length from eight to thirty millimeters, all have nine 

 segments. 



Bumastus billingsi sp. no v. 



(Plate LXI, figures i, 2. ) 



Cf. Bumastus trentonensis Emmons, 1842, Geology New York, Report 



of Second District, p. 390, fig. 1. 

 Cf. Ihcznus trentonensis Hall, 1847, Paleontology New York, Vol. I, 



p. 230, pi. 60, fig. 5. 

 Cf. Bumastus orbicaudatus Clarke, 1897, Paleontology Minnesota, Vol. 



Ill, p. 722, fig. 36. 



The history of the species Bumastus trentonensis Emmons and 

 Illcenus trento7iensis Emmons has been given by Clarke (Paleontology 

 of Minnesota, 1897, p. 718), but apart of it will have to be repeated 

 in order to explain the names here adopted for the species called 

 Bumastus trentone?isis and Illcenus milleri by authors and collectors. 



