SILURIAN BRACHIOPODA . 37 



Rhynchonella neglecta, Hall, 1852 



Plate IV, Figs. 3, 6-8 



, Hall. Twenty-eighth Ann. Rapt. N. Y. State Mus. Nat. Hist., p. 162, pi. 26, figs. 1-6. 



1879. 

 , Hail. Eleventh Ann. Rept. State Geol. Indiana, p. 305, pi. 26, figs. 1-6; pi. 27, fig. 



3. 1882. 



For a species so abundant as this in the Waldron fauna, the diagnos- 

 tic features are retained with unusual persistence within very narrow limi- 

 tations. Unlike its associate, R. mdianensis, which it almost equals in 

 numerical representation, there are no well-established and perduring 

 variations from the normal adult form, and our observations are therefore 

 limited to an essentially unvarying phase. 



SPECIFIC CHARACTERS 



Mature Form (plate 4, figs. 8, 8a). Shell small, transversely sub- 



ovate ; umbo scarcely prominent. Cardinal slopes long and flattened, 

 rounding to the anterior margin which is nearly straight. 



Ventral valve with the umbo elevated and slightly incurved at the tip, 

 overhanging an elongate subtriangular foramen. Umbonal region slightly 

 convex, the convexity extending for one-third the length of the shell ; thence 

 forward the shell is rapidly depressed medially to form a deep sinus, which 

 makes a high quadrangular extension on the margin ; lateral portions 

 depressed. 



Dorsal valve with the umbo low and inconspicuous ; apex concealed 

 within the foramen of the opposite valve. The shell becomes rapidly ele- 

 vated medially to form the fold, the latera being full and convex. 



Surface covered with regular, sharp and prominent plications, which do 

 not vary in number at normal maturity, and which, in the growth of the 

 shell, are increased only from the cardinal margins. Of these plications, 

 the fold bears four, the sinus, therefore, three, and each of the latera, five, 

 those nearest the cardinal margins being obscure. This makes in all for the 



