CAMBRIAN TRILOBITES WALCOTT 25 



Pvgidium large, mocleratel}' convex ; anterior margin slig^htly 

 arched, so as to join with the posterior segment of the thorax ; 

 ] posterior outline semicircular; axial lobe convex and about two- 

 thirds the length of the pygidium; it is divided into seven transverse 

 rings and a terminal section by trans\'«rse furrows ; the pleural 

 lobes slope gently from the axial lobe to the lateral and posterior 

 margins ; their entire surface is marked by the anchylosed seg- 

 ments, which are similar in appearance to the thoracic segments. 

 excejDt that their backward curvature increases until the ])leurge of 

 the posterior segments are nearly parallel to the axis of the 

 pygidium ; the furrows and narrow ridges from the terminal seg- 

 ment of the axis extend backward with a slight inward curvature ; 

 all furrows and ridges terminate just within the outer margin in the 

 same manner as those of the thoracic pleura. 



Surface with relativel}' large granules on all parts of the dorsal 

 shield. 



Dimensions. — A dorsal shield 7.25 mm. in length has the fol- 

 lowing dimensions : 



Cepbalon : mm. 



Length 1 . 73 



Length of glabella i • 50 



Width 2 . 50 



Width of glabclhi i .co 



Thorax : 



Length 3-75 



Width 4-00 



Width of axial lobe at sixth segment 8.00 



Width of pleural lobe i . fco 



Pygidium : 



Length i • 7? 



Width at anterior margin 3 • .-0 



Or.SF,RV.\TioNS. — This is a very rare species, as only one nearly 

 entire specimen is known : this has the pygidium displaced and the 

 free cheeks are missing. The combination of characters found in 

 several genera is shown (a) in the cranidium, in which the glabella 

 is like that of Oryctoccphalus Walcott ; (b) in the thorax, which is 

 not unlike that of Olcniis Salter; and (c) in the pygidium, which 

 suggests in relative size and form the pygidium of Batliyurisciis 

 Jwzi'cUi Walcott.^ Among the associated fossils are Micromitra 

 (Iphidclla) paiuiula (White), Ptychoparia piocJiciisis Walcott, 

 Ptyclwparia cordillercc (Rominger), OryctoccpJiaUis rcynoldsi 

 Reed. ZacantJwidcs idaJwensis, new species, and Batliynriscus 

 Jimcclii A\'alcott. 



' Walcrtt. t886. Bull. U. S. Geo]. Survey, Xo. 30. p. 2ifi. 



