LOWER HELDERBERG ROCKS. 337 



portion above and below nearly equal, and the extremities bending 

 rather abruptly downwards. 



Pygidium triangular, transversely convex ; the posterior extremity acute, 

 attenuate ; the axis a little depressed towards the lower extremity, 

 which rises in strong relief above the border below. The axis is gra- 

 dually attenuate, the width at the posterior extremity being about 

 one-third as great as at the anterior extremity, which is about five- 

 eighths as wide as the greatest width of the lateral lobe at its upper 

 margin : its outline is curved and sometimes scarcely carinate, the 

 latter feature more often seen in the casts. The number of articulations 

 in the axis is seventeen ; and in each of the lateral lobes are eleven 

 to thirteen ribs, which are little wider than the furrows which separate 

 them ; the whole bending downwards towards the outer extremities, 

 and uniting in a thickened border. Each rib of the lateral lobe is 

 marked by a longitudinal groove parallel with the margins, and a little 

 nearer to the upper than the lower margin. 



Surface granulose, the granules being somewhat stronger on the more 

 prominent parts of the head and in front of the eyes, while on the 

 thorax and pygidium there is usually a stronger row of granules along 

 the lower margins of the articulations. The granulose marking, how- 

 ever, is subject to considerable variation, either from accidental or 

 other causes ; and some specimens of the pygidium present a fine 

 granulose texture, visible only under a lens. 



The original specimen, a pygidium, from which Dr. Green described his species, 

 lies before me; while I have ten others in various stages of growth, one of them 

 four times as large as this one, all preserving seventeen articulations in the axis, 

 the larger one alone having thirteen ribs in each of the lateral lobes. The specimen 

 described by Dr. Green has the axis flattened from compression, but this is not the 

 true character of the animal : the prolongation of the caudal extremity is likewise 

 worn off, so that it has been described as terminating in an obtuse tip. 



The specimen mentioned by Dr. Green as from the Genesee river is doubtless the 

 pygidium of Dalmania limulurus = Phacops limulurus ( Paleontology of New-York, 

 Vol. II ), since the shaly limestone of the Lower Helderberg group is not known to 

 extend so far west. 



