339 



Occurrence. ELBROOK LIMESTONE. Small quarry on the eastern out- 

 skirts of Waynesboro, Pennsylvania. 

 Collection. U. S. National Museum. 



Family MESONACIDAE 



Genus OLENELLUS Hall 



OLENELLUS THOMPSON! (Hall) 



Plate XXIV 



Olenus thompsoni Hall, 1859, 12th Ann. Kept. New York State Cab. Nat. 



Hist., p. 59, fig. 1, p. 60. 

 Olenellus thompsoni Walcott, 1886, U. S. Geol. Surv. Bull., No. 30, p. 167, 



pi. xvii, figs. 2, 4, 9; pi. xxii, fig. 1; pi. xxiii, fig. 1. 

 Olenellus thompsoni Walcott, 1890, 10th Ann. Kept. U. S. Geol. Surv., 



pi. Ixxxii, figs. 1, la; pi. Ixxxiii, figs. 1, la. 

 Olenellus thompsoni Walcott, 1910, Smithsonian Misc. Coll., No. 53, p. 336, 



pi. xxxiv, fig. 9; pi. xxxv, figs. 1-7. 



Description. " General form ovate, the length and breadth being 

 nearly as six to five. Head broad lunate, with the postero-lateral angles 

 much extended ; the width from the center to the outer margin of the eye 

 almost equal to the width of the cheek. Eyes (which are much crushed 

 in the specimen) elongate semi-oval, equal in length to the space between 

 the anterior angles and the frontal margin; glabella distinctly lobed, 

 narrower in front. 



" Thorax with the lateral lobes about once and a half as wide as the 

 middle lobe, consisting of 14 articulations, the third one of which is much 

 longer than the others, and curving downwards with an extension reach- 

 ing as far as the line of articulation of the seventh rib. The posterior 

 articulations are bent abruptly backwards, so that the free extremities are 

 parallel with the axis. Pygidium small, pointed, without visible rings 

 and having a narrow ridge running down the center." Hall, 1859. 



Occurrence. ANTIETAM SANDSTONE and TOMSTOWN LIMESTONE. 

 Near Smiths burg, and at Eakles Mills in Washington County, Maryland. 

 The type fossil of the Lower Cambrian in the eastern United States and 

 Canada. 



Collections. Maryland Geological Survey, U. S. National Museum. 



