No. 89.] Si 



the beak of the dorsal valve. The interior of the ventral valve shows a short V- 

 shaped process attached to the median septum for a distance of one-third the 

 length of the shell from the beak, the septum continuing below the middle of the 

 shell. Surface punctate : substance of the shell lamellose, prismatic, brittle. 

 This species resembles P. Occident alis of the Iowa Geological Report; but the 



plications are more numerous and conspicuous, and extend farther towards the 



beak. The texture and surface markings are similar. 



Geological formation and locality. Shales of the Hamilton group, in numerous 



places in Western New-York. 



RHYNCHONELLA SAPPHO ( n.s.). 



Shell transverse, gibbous or ventricose; cardinal slopes nearly straight or a little 

 concave, and regularly rounded below. Dorsal valve very gibbous, regularly 

 convex transversely ; about six to eight plications elevated upon the mesial fold, 

 which is distinctly but not prominently raised towards the front. Ventral valve 

 flabelliform, depressed convex, a little gibbous near the beak, flattened in the 

 middle near the front, and abruptly bent up at the margin for the width of the 

 sinus; the margins on each side of the sinus a little recurved. Surface marked 

 by twenty to twenty-four plications, of which, in old shells, usually six or seven 

 mark the mesial fold and sinus. The middle plications are flattened or grooved 

 towards the margin, while the lateral ones are more angular, and become obso- 

 lete on the cardinal line. Surface crossed by fine close undulating concentric 

 lines. 

 This is a very beautiful and symmetrical species, and, in full-grown individuals, 



has the length and width as seven to eight. In young shells, there are not more 



than four plications on the sinus and fold. 



Geological formation and locality. In limestone of the Marcellus shale : Near 



Leroy, N.York. 



RHYNCHONELLA HORSFORDI (n.s.). 



Shell, in full-grown specimens, transverse, gibbous, length and width about as 

 five to six or six to seven ; front nearly straight, abruptly and shortly pointed 

 at the beak; cardinal slopes concave; sides rounded. Dorsal valve very gibbous, 

 sloping abruptly to the beak; mesial elevation defined below the middle of the 

 valve. Ventral valve moderately convex, flattened and incurved in front, forming 

 a distinct mesial sinus; lateral portions gently curving to the margins. Surface 

 marked by twenty to twenty-four well-defined angular plications, about five of 

 which mark the mesial sinus and fold. The plications limiting the sinus are more 

 sharply elevated than the others, concentrically marked by fine undulating striae. 

 This species differs from JR. sappho, in the stronger and more angular plications 



which characterize all stages of growth : it is also smaller at maturity, and more 



straight in front. 



Geological formation and locality. In shales of the Hamilton group, at nume- 

 rous places in Western New-York. 



