CRUSTACEA. L15 



Pleura flattened near the axis for less than one-third their width, thence 

 deflected, in a rather abrupt curve, to the lateral margins. Segments strongly 

 sulcate over the fulcrum, the anterior limb being sharply angulated and he- 

 coming rapidly obsolete upon the beveled articulating surfaces. 



Pygidium rounded sub-triangular, slightly produced at the extremity; length to 

 width as 1 to 1.5. Surface convex, sloping evenly to the lateral and poste- 

 rior margins. 



Axis narrow, its width on the anterior margin being about one-fourth the 

 width of the shield; longitudinally arched and slightly angulated poste- 

 riorly along the median line; tapering evenly to an obsolete termination at 

 the border. Composed of eleven or twelve flattened, transverse annulations. 



Pleura, broad and evenly rounding to the margins ; bearing from eight to 

 eleven annulations, each being grooved by a faint impressed line, which 

 is most distinct near the axis and toward the margins. The border is 

 moderately broad and thickened, widening to the posterior extremity, the 

 annulations, with the exception of the first three or four, becoming extinct 

 upon reaching its inner margin. 



Surface smooth or minutely granulose. 



Dimensions. The only entire individual observed measures as follows : 



Body. Cephalon. Thorax. Pygidium. 



Length 20 mm. 7 mm. 7 mm. 6 mm. 



Width 11mm. 11mm. 10 mm. 9 mm. 



Observations. The specimen upon which this species was founded is a beauti- 

 fully preserved example, which was obtained many years since from the late 

 Professor S. S. Haldeman, of Columbia, Pa. Its locality is not known further 

 than that it is from the south-western part of the State of Pennsylvania. It 

 is preserved in a compact, fine-grained, chocolate-brown limestone, undoubtedly 

 of the age of the Hamilton group of the State of New York, since isolated 

 cephala and pygidia of the same species have been found in the Goniatite lime- 

 stone of the Marcellus epoch in this state. It is a very distinctly defined 



