CRUSTACEA. 3 



along the axial line. Upon the pleura' the segments are grooved, the ante- 

 rior limb of each being small and becoming obsolete on the beveled planes 



of articulation. 



Pygidium spherically sub-triangular, truncate and strongly emarginate on the 

 posterior bonier. Length to width as 1 to 1.3. 



Axis broad, rapidly narrowing and ending abruptly in a blunt elevation 

 within the posterior border; its terminal portion is flattened above, and 

 from its apex a low ridge passes over the post-axial area. Axial anima- 

 tions six. 



Pleura broad, and abruptly deflected. Anterior margins strongly beveled 

 by the articulating planes, and meeting the lateral margins at an angle of 

 ninety degrees. Pleural annulations lour, flattened near the margin and 

 sometimes showing a tendency to bifurcato . 



Hypostoma sub-quadrate; lateral margins strongly incurved, posterior border 

 emarginate. Anterior angles auriculate, margins thickened and slightly 

 reflexed ; centrum elevated, sharply carinate. 



Surface Ornamentation. The east of the external surface of a nearly entire 

 individual, which served as Dr. Green's type of the species, shows faint 

 tubercles over the glabella and free cheeks, but these appear to be obsolete 

 upon the occipital annulation and the rest of the body. 



Dimensions. The smallest individual of this species which has been 

 observed is the type specimen. It affords the following measurements: 

 Length 63 mm.; width 45 mm. The largest individual observed measures: 



i: ily. lalon. Thorax. Pygidium. 



Length 130 mm. 30 mm. 75 mm. 25 mm. 



Width 75 mm. 75 mm. 70 mm. 30 mm. 



Fragments of still larger individuals are sometimes found, a single pygidium 

 from the Upper Helderberg limestone of the Kails of the Ohio measuring 

 45 mm. in length and 35 mm. in width, indicating an individual of quite 



