Kathbun.1 22 [May 15, 



Devonian sandstone, Rio Curua and doubtfully on the Msecurvi. 

 (Geol. Comni., 1876.) 



Chonetes, sp. indet. 



There was obtained from the Devonian sandstone of the Rio Mae- 

 curu a very small, smooth form of Chonetes, represented by only two 

 specimens, which do not appear to belong to any of the species al- 

 ready described in this paper. The specimens are not, however, well 

 enough preserved to permit of their being accurately determined. 

 Orthis Nettoana Rathbun. 



Bull. Bufl". Soc. Nat. Sci., I, no. 4, p. 247, pi. x, 1874. 



The finding of larger, more perfect specimens of this species in 

 1876 necessitates the following new description. 



Shell small, with the valves subequally convex, or with the dorsal 

 valve depressed and much less prominent than the ventral. Ventral 

 valve subcircular or transversely subelliptical in outline; dorsal 

 valve subelliptical. Greatest width of the shell near the middle; 

 length of the hinge line equal to two-thirds the greatest width, or a 

 little less; cardinal extremities rounded. Shell marked with radi- 

 ating, raised lines. 



Ventral valve quite convex and most prominent at, or just poste- 

 rior to, the middle, whence the slope is very gentle toward the beak, 

 but ratlier rapid toward the front. Surface of the valve curving regu- 

 larly from side to side, or descending from the median line to the lat- 

 eral margins with little curvature; curve from the beak to the front 

 sometimes very regular. Cardinal extremities broadly and regu- 

 larly rounded. In the broader forms the anterior margin is straight- 

 ened along the front, or even slightly indented. Hinge area high, 

 broadly triangular, with a broad triangular fissure, which sometimes 

 occupies half of the area or more. Beak acute, but not extending 

 beyond the line of the cardinal margins and scarcely incurving. 



Dorsal valve moderately convex, sometimes depressed-convex, and 

 generally with a broad, shallow, undefined median depression, which 

 commences near the beak, and broadens rapidly toward the front, 

 where its width equals one-third to one-half that of the valve. Beak 

 depressed; hinge area narrow. 



Dental lamellae of the ventral valve, more or less separated at the 

 beak, and extending forward, nearly parallel with one another, or 

 diverging slightly. They are generally curved, their concave sur- 

 faces facing inwards. Distance between the lamellae equal to about 

 one-fifth or one-seventh the width of the valve; their length, one- 



