CRUSTACEA. 157 



Telson and appendages unknown. 



Dimensions. The single specimen from which this description is drawn 

 is somewhat curved to the left toward the posterior extremity, and has a 

 length along the axis of 97 mm. The body measures 39 mm. in width at 

 the first somite, 41 mm. at the fifth, and 20 mm. at the eleventh. The 

 fragment of the natatory appendage measures 50 mm. in length, of which 

 the proximal joint measures 30 mm. 



Observations. This specimen, although in an imperfect condition, is suffi- 

 ciently well-preserved to allow of description and identification, and is of 

 especial interest as the only representative of the genus known to occur in 

 this geological formation. 



Distribution. Chemung group. In the sandstones at Warren, Warren county, 

 Pennsylvania. 



EURYPTERUS PROMINENS. 

 PLATE XXVII, FIGS. 3, 4. 



Euriipterii.i promineiis. Hall. Proc. Amer. Assoc. Adv. Science, vol. xxxiii, \>. -120. 1884. 



Cephalon sub-quadrate in general outline ; anterior margin broadly circular, 

 curving regularly to the lateral margins, which are parallel ; posterior mar- 

 gin transverse, bending slightly forward. Surface convex, sloping more 

 abruptly in front than laterally ; flattened above over a triangular area, 

 bounded by a faint transverse ridge joining the centers of the eyes and two 

 oblique ridges passing from the posterior angles of the eyes to near the 

 posterior margin, meeting in the axial line. 



The eyes are large, lunate or obliquely ficulate, distant, situated near the 

 antero-lateral margins. The surface of the cephalon beneath their outer 

 edges is depressed. The ocelli are represented by two minute pits lying 

 near the axial line and in a transverse line joining the posterior extremities 

 of the eyes. These are separated from each other by a distance of 2 mm. 

 On the postero-lateral slopes are two oblique linear depressions having the 

 lower extremities directed inward. These are each situated about half way 



