.196 PALiEONTOLOGY OF NEW-YORK. 



Thokax 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 po- 

 sterior articulations are bent abruptly backwards, so that the frea 

 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. 



Olenus thomp§oni. 



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



