TEREBRATULID^ OP THE HAMILTON GROUP. 391 



Geological formation and locality. In strata of the age of the Upper Hclder- 

 berg or Hamilton group, at Waterloo, Iowa. Received froiu Mr. O. St. John. 



Terebratula iiavicella (n. s.). 



PLATE LX. 



Shell elliptical, subnaviform, arcuate on the ventral side, slightly lingui- 

 form in front. 



Ventral valve extremely arcuate, flattened or subsinuate towards the 

 front, depressed convex in the middle, and a little prominent on the 

 umbo : beak extended, recurved and truncated by a rounded fora- 

 men almost in the plane of the longitudinal axis; deltidial pieces 

 conspicuous. 



Dorsal valve prominent along the middle, nearly straight from the base 

 to the umbo, sloping somewhat abruptly to the sides. 



Surface marked by fine concentric striae which are sometimes crowded 

 into stronger ridges. Structure distinctly punctate. 

 On cutting down the dorsal side, there are some obscure indications 



of short crura which are not united below : but this is a common condi- 



tion, the connecting portion of the loop having often been broken away. 



Geological formation and locality. This species occurs at Eockford, Iowa, iu 

 bods of the age of the Hamilton or Chemung group of New- York. Received from 

 Mr. O. St. Joilv, and collected at the same place by Mr. R. P. "Whttfield. 



Terebratula simulator (n. s.). 



PLATE LX. 



Shell elongate-ovate or spatulate, moderately convex ; the beak of the 

 ventral valve much extended, and truncated almost rectangularly to 

 the longitudinal plane. 

 •A specimen received from Dr. Rceminger, collected in the Hamilton 



group at Widder, Canada West, shows the crura extending for about 



half the length of the dorsal valve ; but the connecting portion of the 



loop is not preserved. 



