RHYNCHONELLIDJE OF THE HAMILTON GKOUP. 
039 
Rliyiichonclla (Stenocisma) horsfordi. 
PLATE LIT. 
Rhynchonella horsfordi : Hall, Thirteenth Report on the State Cabinet, p.87. 1860. 
Shell, in full-grown specimens, transversely subelliptical; rostral por¬ 
tion sometimes a little extended; front nearly straight or broadly 
rounded ; length and width about as five to six or seven. Young shells 
ovoid subtrigonal. 
Ventral valve moderately convex, flattened and incurved in front; a 
slightly depressed sinus, appearing about the middle of the length, 
which is flat in the bottom and curving abruptly upwards in front : 
beak moderately extended, abruptly acute and usually but little 
incurved. 
Dorsal valve very gibbous in old shells, sloping abruptly to the beak; 
depressed-convex in young shells. Mesial elevation defined below the 
middle of the length. 
Surface marked by about fifteen or sixteen to twenty-four well-defined 
angular plications, of which four to six or seven mark the mesial sinus 
and fold, which are deeply bifurcated in front. On the sides and 
towards the cardino-lateral margins of the shell the plications are 
less angular : concentrically marked by fine undulating striae, which 
are seen towards the front, but rarely on other parts of the shell. 
* The size varies from a quarter of an inch in length and five-sixteenths 
in width to nine-sixteenths in length and thirteen-sixteenths of an inch 
in width, according to age. 
A specimen of half an inch in length from the limestone of the Falls 
of the Ohio, sent to me by Dr. Knapp, which I refer to this species, presents 
a strong interrupted line of growth above the middle’ and has about 
twenty-eight plications visible, of which four mark the sinus and five 
the mesial fold. 
This species differs..from the R. (/S'.) sajpjplio in the finer, more numerous and 
more angular plications ; and is usually a much smaller shell. It can scarcely be 
compared with any of the devonian forms of Great Britain, having only a remote 
