341 



Description. Pygidium quite convex, with a strong triangular spine 

 behind. Axis conical ; occupying less than one-third of the width, strongly 

 convex and clearly outlined by the dorsal furrow; apex of axis rounded 

 and with four or five rings. Side lobes, with four or five broad and short 

 ribs, which extend about half way to the margin. A smooth, slightly con- 

 vex border all around extends backward, and forms the terminal spine. 



The well-marked triangular terminal spine of this species causes its 

 identification to be extremely easy. So far only the pygidium of the 

 species has been discovered. 



Occurrence. BEEKMANTOWN LIMESTONE (Ceratopea zone). Vicinity 

 of Halfway, Maryland. The types are from the Canadian rocks of New- 

 foundland. 



Collection. U. S. National Museum. 



Family OLENIDAE 



Genus TRIARTHRUS Green 



TRIAETHEUS FISCHEEI Billings 



Plate LI, Fig. 16 



Triarthrus fischeri Billings, 1865, Pal. Foss., vol. i, Geol. Surv. Canada, 

 p. 291, text fig. 280. 



Description. A small cephalon of Triarthrus found in the Sinuites 

 bed at Chambersburg, Pennsylvania, is so similar to Billings' illustration 

 of T. fischcri copied on pi. LI that it is believed to represent the same 

 species in spite of the supposed difference in their geological horizons. 

 It is possible that the strata in which the type of T. fischeri occurred are 

 younger than now believed, and again it would not be surprising if this 

 species, like many others, is repeated at several horizons in the geologic 

 column. 



Triarthrus fischeri differs conspicuously from T. becki in the absence of 

 tubercles along the median line of the axis. The types are from the 

 Chazyan of Newfoundland. 



Occurrence. MAETINSBURG SHALE (Sinuites bed). Chambersburg, 

 Pennsylvania. 



Collection. U. S. National Museum. 



