TEREBRATULIDiE OF THE HAMILTON GROUP. 
391 
Geological formation and locality. In strata of the age of the Upper Heidel¬ 
berg or Hamilton group, at Waterloo, Iowa. Received from Mr. 0. St. John. 
Tereliratula iiaviceila (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 Rockford, Iowa, in 
beds of the age of the Hamilton or Chemung group of New-York. Received from 
Mr. O. St. John, and collected at the same place by Mr. R. P. Whitfield. 
Tereforatula 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. 
