INVERTEBRATES. 115 



Length of the largest individual seen % inch ; breadth about 

 the same: length of hinge line about % of an inch. This form 

 differs from the A. monroenxix in its smaller size and nearly 

 smooth surface. Named in honor of my friend Henry Talbot, 

 Ksq.. of Waterloo. 



Position and locality: Same as the last. 



Collector. A. H. Wort hen. 



No. 2510 of the Illinois State Museum. 



AvirrLopErTEN ELSAHE.vsis. "Worthen. 



PL XXII, Fig. 3. 



Aviculovecteii rlfahpnsit, Worthen, March, 18*4. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 19. 



Shell of medium size, left valve moderately convex, and ex- 

 clusive of the pars presenting a slightly oblique sub-trigonal 

 outline: hinge line equaling about two-thirds the greatest width 

 of the valve: basal outline forming a somewhat oblique semi- 

 circular curve; posterior margin straight from the base of the 

 ear to the basal curve where the valve attains its greatest width. 

 Anterior margin regularly rounded; anterior ear small and ob- 

 tusely angular: posterior ear large, triangular, and sharply 

 pointed at the extremity. Surface of the ears apparently with- 

 out ornamentation. Surface of the shell ornamented with nu- 

 merous radiating cost.e that increase by implantation, and are 

 somewhat variable in size, being usually about equal to the in- 

 tervening Spaces. 



Length I 1 " u - inches: breadth I 11 / inches: length of hinge 

 line I 1 * inches. 



Position and locality: Kiuderhook group? half a mile above 

 Elsah. Jersey county. 111. 



Collector. A. H. Worthen. 



No. 2506 of the Illinois State Museum. 



AvifTLOPEtTEN CHESTEREXSIS. Woi'theU. 

 PI. XXI, Pig. 5-Sa. 



Aviculopecten chesterensis, Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 20. 



Shell above the medium size, nearly equilateral, broadly ovate 

 or semi-circular in outline, a little wider than long; left valve 

 moderately convex, beak depressed and extending slightly be- 



