Geraurus pleurexanthemus. 161 



vated projecting surface : outer surface granulated. Interior concave, the 

 margin a reflected, edge or " doublure." Anterior margin a semi-lunate 

 curve, attached to the anterior margin of the glabellar depression by the 

 hypostomatic suture. 



Thoiiax. Each segment may be divided into three parts, viz. : 1, 

 Axial groove; 2, Thoracic pleura? ; 3, Free pleurae. The axial groove 

 consists of the axial ring, anterior " articular fold," and a reflexed pos- 

 terior articular margin. The " articular fold " rests upon the thin edge of 

 the reflected posterior articular margin of the next anterior segment. 

 The anterior margin of the "articular fold" describes a curve from the 

 anterior lateral extremities of the axial ring, forward into the axial 

 groove, nearly concealing the preceding axial ring. The anterior margin 

 of the axial ring is thickened, as a base for the articular fold, and also as 

 the base of a pair of processes extending from the lateral extremities 

 obliquely backward one-fourth the distance across the axial groove. Each 

 process is a plate-like projection, surmounted at its upper extremity by 

 a small knob-like elevation. 



Thoracic pleura? of each segment divided by diagonal ridges into two 

 triangular depressions upon each pleura, separated from the axial groove 

 and circular cavities, by short transverse ridges. Circular cavities situ- 

 ated between the triangular depressions and the free pleura? ; they are 

 deeper than the triangular depressions. Anterior and posterior margins 

 of the pleura? parallel. 



The free pleura? curve outward and backward, terminating in falcate 

 extremities. The hollow interior of each opens into the thoracic cavity 

 at the inner extremity, which has upon its upper margin a crescent- 

 shaped surface or slight sulcus. The whole thorax narrows posteriorly. 



Pygidium: semicircular, concave, and surrounded by a strong "doub- 

 lure," which has a smooth subcrescentiform surface upon each anterior 

 lateral margin. Anterior lateral margins parallel to those of the posterior 

 segment of the thorax. The articular fold rests upon the axial ring of 

 the posterior segment. The pygidium is composed of four anchylosed 

 segments ; the anterior one, penetrating the." doublure " and lateral mar- 

 gins, is produced into long curved spines. Four pair of axial processes 

 project into the axial depression; the anterior pair well developed, the 

 posterior pair as rudimentary tubercles under the " doublure." Upon the 

 posterior surface of the anterior anchylosed segment, there are two 

 minute oval openings, one on each side of the median line, the longer 

 axis extending obliquely upward and backward. 



Formation and locality, upper third of the Trenton Lime- 

 stone, Trenton Falls, Oneida Co., N. Y. 



