INVERTEBRATES. Ill 



Hinge line about two-thirds as long as the valve with a wide 

 cardinal area nearly equaling it in length. Basal area regularly 

 curved from the truncated anterior margin to the posterior ex- 

 tremity. 



Anterior muscular scar, situated close to the anterior extrem- 

 ity, and about midway from the beak to the basal margin. 

 Dorsal scar about twice as large a.s the anterior one, and 

 located midway between the beak and the posterior extremity. 



Surface markings unknown. 



Length 2% inches: height about 1* 1H inches. 

 Position and locality: Chester limestone, Chester, 111. 

 Collector, A. H. Wort hen. 

 No. 2590 of the Illinois Sun.- Museum. 



GENUS PINNA, Linnaeus. 

 PINNA SAN< TI-LVDOVICI. Worthen. 



PL XIX, Fig. 6. 

 Pinna St. Ludorici Worthen, 1883. GeoL Survey of Illinois Vol. VII, page 336. 



Shell comparatively short, lanceolate; length about twice MS 

 much as the greatest width. Section ovate-elliptical, valves 

 traversed longitudinally by about twelve to fifteen rather strong 

 ribs that are most prominent on the middle portion of the shell, 

 and become obsolete towards the ventral and dorsal margins. 

 Spaces between the ribs narrow, beak unknown. Angle formed 

 by the dorsal and ventral margins about 40. 



This rare species is only known from a single specimen pre- 

 served in chert, with both extremities wanting. It may be 

 readily distinguished from P. inisfiouriensix. of the Chester lime- 

 stone, and from any other species known in our carboniferous 

 rocks, by its shorter form and greater proportional width. 



Position and locality: St. Louis limestone. Monroe county, 

 Illinois. 



Collector, H. F. Henckler. 



No. 2490 of the Illinois State Museum. 



