NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 93 



Strophomena rkomboidalis'Wahl. Numerous excellent examples of the middle-silurian type, 

 showing both external and internal characters. We shall probably discover grounds here- 

 after for a division of this nominal species. 



STREPTORHYNCHUS King. 



Streptorhynehus hemiaster W. and M. 



Plate II. figure 10. 



Shell small, semicircular, hinge-line the longest diameter; in some specimens slightly 

 auriculate. Ventral valve depressed-convex, with an abruptly elevated beak, truncated by 

 the area on the cardinal side, and gently twisted toward the right ; area triangular, nearly 

 at right angles with the plane of the shell, its fissure closed by a convex pseudo-deltidium 

 of one piece. Surface marked by straight, sharp, radial costse which double in number by 

 implantation at about one third the distance from the beak to the margin, and double 

 again nearer the margin. Dorsal valve flat, appearing, in the condition in which we find 

 the specimens, to be constituted by a series of radiating, rounded, threadlike ribs which 

 show most conspicuously on the inside, the principal ones extending from the beak to the 

 margin, and increasing in number by implantation. 



Length of hinge-line .38 inch ; length of shell .23 inch ; height of area of ventral valve 

 .10 inch. 



Atrypa reticularis Dalm. sp. Abundant, often exhibiting the internal structure in a 

 beautiful state of preservation. 



Atrypa nodostriata Hall. Pal. N. Y., II. 272, pi. Ivi. 2. Common, in an imperfect condition, 

 but in some instances exhibiting the spiral armature. 



Trematospira Matihewsoni McChesney. New Paleozoic Fossils, p. 71. 



Athyris nitida Hall sp. Pal. N. Y., II. 268, pi. Iv. 1, 2. Occurs in the typical form, and 

 the variety oblata Hall. 



Nucleospira sp ? A small pyriformly ventricose species, exhibiting the internal struct- 

 ure, but too imperfect for description. We suspect this is the species referred to by Hall 

 (Iowa Rep. p. 73, note), as a Spirigera, occuring at Leclaire, Iowa, in great numbers. 



SPIRIFERA Sow. Martinia McCoy. 

 Spirifera (Martiaia) similior W. and M. 



Outline subcircular, transverse diameter greatest, gently rounded from the sides to the 

 cardinal margin ; hinge-line about two thirds the greatest width of the shell. Ventral 

 valve ventricose, gently convex to the very margin on all sides, deepest at about one third 

 the distance from the beak to the anterior margin ; beak incurved and slightly surpassing 

 the hinge-line, overhanging a large triangular fissure, without noticeable area. A trace of 

 a mesial septum extends from the beak one third the length of the valve. In some speci- 

 mens no sinus whatever can.be discerned ; in others, a shallow and obsolete one ; in others 

 still, one or two inconspicuous undulations each side of a mesial sinus. Surface of cast 

 otherwise smooth, or with numerous fine concentric wrinkles. In one specimen faint radi- 

 ating lines may be doubtfully detected. 



