1878.] 23 [Rathbun. 



fourth that of the valve, or slightly more. The socket plates of the 

 dorsal valve extend in about the same direction, or diverge slightly 

 more, but they are not as long as the lamellfe. Cardinal process 

 not preserved. 



Shell marked with fine, closely arranged, radiating raised lines, 

 which are of about the same width as the depressions separating 

 them, or slightly broader. They appear to increase in number most- 

 ly by intercalation. An elliptical specimen measures, length 15 mm., 

 breadth 21 mm. ; a nearly circular one, length 15 mm., breadth 

 16 mm. 



This species belongs to the type of Orthis lentlcularis, of the Upper 

 Helderberg Group of New York, though specifically it does not re- 

 semble any N. American form. It is represented in the Devonian 

 sandstone of Erere by only a few very small specimens; many speci- 

 mens of larger size were, however, obtained from the Rio Msecuru. 

 Two or three small specimens were found in the sandstone of the 

 Rio Curua. (Morgan Exs., 1870 and '71; Geol. Comm., 1876.) 

 Orthis Harttii, sp. nov. 



Shell of medium to large size, transverse, subplano-convex, or with 

 the ventral valve slightly convex on the umbo. Dorsal valve broad- 

 ly subovate in outline; ventral, transversely subelliptical. Hinge 

 line short, equal to about one-third to one-half the width of the 

 shell. Surface marked with fine, radiating raised lines. 



Ventral valve broadest near the middle; proportions of length to 

 breadth about as 2 to 3, or length and breadth nearly equal. There 

 is an undefined, shallow, and rather narrow, median sinus, which causes 

 a slight indentation in the anterior margin of the valve. It com- 

 mences some distance forward of the beak, and is often scarcely de- 

 veloped at all. Beak of medium size, prominent and acutely pointed, 

 and projecting a short distance beyond the hinge line. The sur- 

 face of the valve curves up rapidly from the hinge line toward the 

 middle of the valve, but all the median and anterior portion of the 

 valve is broadly flattened, and at times depressed to form the sinus. 



Dorsal valve moderately convex, and broadest just anterior to the 

 middle, the margins curving rapidly outward and forward from the 

 beak to the point of greatest width, in front of which the margins 

 form a regular curve, so that, as a rule, the valve has quite a perfect, 

 broad-ovate outline. Along the median line the valve is usually 

 broadly, but not strongly, elevated in an undefined prominence, 

 reaching from near the beak to near the front of the valve. This el- 



