Tkii.or.nEs oi-- the Cha/v Limkstonk. 377 



Thorax. — Total number of thoracic segments not known. Axis 

 high, about one third the width, tapering gradually. Segments 

 narrow, turning down sharjily at the sides. Segments on pleura 

 deeply grooved. 



Pygidiiim. — Pygidium somewhat triangular in some s])ecimens, 

 while others are so broad as to give a rather rounded outline. Axis 

 high, outlined by deej) furrows. It tapers somewhat to about the 

 middle of the pygidium, then remains the same breadth to its end. 

 It does not reach to the end of the i)ygidium and the space behind it 

 is smooth. Axis has from ten to twelve sinuous annulations and the 

 plur^e show six to eight ribs which are single for a part of their length 

 and then bifurcate. They reach almost, but not cjuite to the margin. 

 In some specimens the ribs bifurcate (juite close to the axis, while on 

 others the rib remains single more than one fourth its length. On 

 some si)ecimens the anterior portion of the rib continues much nearer 

 the margin than the posterior part, thus forming a very characteristic 

 appearance. On the axis are small pits between the annulations, 

 which give a double curve to each side of each annulation. Both 

 cephalon and pygidium are finely tuberculated. 



Measurements. — ^A ce])halon : length S.5 mm.; width 17 mm.; 

 width between eyes, 7.5 mm. Another: length 7 mm.; width 12 

 mm.; betw^een eyes, 8 mm. A pygidium : length 10 mm.; width 13 

 mm.; axis 3.5 mm. wide in front. Another: 8 mm. long; 10 mm. 

 wide; axis 2.5 mm. wide in front, 6 mm. long. 



Locality. — .^11 through the Chazy at Valcour Island. I'ype in 

 Carnegie Museum. 



Conclusion. 



In the species which have been described on the preceding pages, 

 all the orders of trilobites are represented, the Hypoparia by two 

 families, two genera and three species, the Opisthoparia by five fami- 

 lies, eleven genera and nineteen species, and the Proparia by three 

 families, seven genera and eleven species. The Opisthoparia are 

 most numerous, both in individuals and species. The Asaphidre, 

 with six genera, fourteen species and countless individuals seem to 

 reach their culmination in these rocks. 



Seven genera, Thalcops, Proctiis, Glaphurus, Cybele, SpJucrocorphe, 

 Spha^rexochiis and Ptefygonietopus make their first apjjearance in Amer- 

 ican faunas in this formation. Three, Bathyurelliis, Glaphurus and 

 Amphion are not found in formations above the Chazy. 



