198 CALIFORNIA ACADEMY OF SCIENCES. 



edge being more elevated, the furrow of the cheek, 

 which meets the posterior cheek furrow at right angles, 

 and the coarser granules which cover the whole surface 

 of the glabella and cheeks. The pygidium can readily be 

 distinguished from Proetus orbitatus by its distinctly flat 

 arch and less length, more breadth, more prominent seg- 

 mentation and narrower axis; also by the granules of the 

 shell on the rings. These are arranged in duplicate rows 

 of nodes, two exterior and two interior rows; those of the 

 exterior are rounded in form and generally located at the 

 end of the rings or at the border of two neighboring rings. 

 Those of the interior rows are triangular, pointed back- 

 wards, and are always connecting with the furrows divid- 

 ing the rings. Analogous impres sions can be seen on 

 well preserved specimens of Proetus orbitatus. 



The Missouri species, although characterized by a 

 smooth tail, belongs to a different group of the Proetidse, 

 with only nine thorax segments. The head is parabolic 

 in form. The posterior angles of the movable cheeks 

 are not prolonged into spines, but obtusely rounded. 

 The eyes are similar in location to those of Phillipsia 

 Sampson/, from the same beds. The thorax has 9 pleura?, 

 with a broad and prominent axis. The pygidium exhibits 

 8 axial rings, with indications of two faintly defined seg- 

 ments on the lateral lobes. The pygidium is slightly longer 

 than broad. One young exhibits this characteristic, but 

 the more mature specimens are much more elongated, 

 giving more characteristic Phillipsial forms to this part. 



The auxiliary impressions cannot be made out on the 

 specimens before us. Altogether, the Missouri species 

 shows affinities with its German prototype, but differs 

 sufficiently to bear the new name of Proetus ? placidus. 



Geological position and locality — Chouteau limestone, 

 Sedalia and Providence, Missouri, from the Cabinet of 

 Prof. Broadhead of Columbia, Mo. 



