RHTXCHONELLID^ OF THE HAMILTON GROUP. 339 



Rh}nclioiieIla (Stenocisma) Iiorsfordi. 



PLATE LIV. 



Rkynchonella hortfordi : 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 ajid 

 towards the cardino-lateral margins of the shell the plications are 

 less angular : concentrically marked by fine undulating striaB, 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 li. (S.) saj)2)Jto in the finer, more numei-ous 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 



