No. 186.] 59 



TMLOBITES 



SHALES OF THE HUDSON-RIVER GROUP. 



The Trilobites most common in the shales of the Hudson-river group 

 are Triarthrus becJdi and Calymene senaria z=. C. blumenbachiil I 

 have likewise described two species of Olenus in the first volume of 

 the Palaeontology of New-York ; but these are rare in most locali- 

 ties of the rocks of this period. 



Some years since, during the progress of the Geological Survey of 

 Vermont by Rev. Z. Thompson, some specimens of Trilobites were 

 obtained from the shales of this age in the town of Georgia; and 

 these were subsequently placed in my hands. The Survey having 

 since passed under the direction of Professor Hitchcock, I postponed 

 the publication of the descriptions, fearing it might not be agreeable 

 to him; but having now not only his approval, but his express desire 

 that I would publish them, I give below the following species, pre- 

 liminary to a more complete description and illustration. 



Olenus thompsoni ( n. s.). 



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 centre to the outer margin of the eye almost equal to the 

 width of the cheek. Eyes { which are much crushed in the specimen) 

 elongate semioval, equal in length to the space between the anterior an- 

 gles 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 fourteen articulations, the third one of which is much 

 longer than the others, and curving downwards with an extension reaching 

 as far as the line of articulation of the seventh rib. The posterior articula- 

 tions 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 centre. 



The description is chiefly drawn from an impression in slate, and a cast 

 made from the same, together with some fragments of the same species. 



Geological position. In the shales in the upper part of the Hudson-river 

 group. 



