LOWER HELDERBERG ROCKS. SSO 



sharp interlocking edges of the front and lateral margins of the valves. 

 These fine striae doubtless represent lines of growth, which have be- 

 come wholly obsolete on other parts of the shell. 



In this species the two plications bounding the sinus of the ventral valve;, and 

 the mesial elevation of the dorsal, sometimes bifurcate towards the beak or middle 

 of the valves, one becoming obsolete on the front. It resembles some of the Bohemian 

 forms which are considered varieties of R. wilsoni, but is a larger and relatively 

 broader shell, the mesial elevation is more distinct, and the general form is less 

 rotund. It is possible that more extensive collections may prove this species to be 

 an extreme variety of the preceding. 



Fig. 3 a, h. Dorsal and ventral views of a well-marked individual of this speoiea. 

 Fig. 3 c, d. Front and cardinal views of another individual. 



Geological position and locality. Shaly limestone of the Lower Helderberg group, 

 Albany and Schoharie counties. 



Rfaynchonella pyrainidata ( n. s.). 



Plate XXXII. Fig. 1 & 2. 



Shell pyramidal, subpentagonal ; outline subquadrangular. Ventral valve 

 nearly flat or depressed convex, more or less abruptly deflected at the 

 margins towards the other valve : beak prominent, flattened, and 

 closely incurved over that of the opposite valve. Dorsal valve very 

 gibbous, declining from near the front towards the beak : beak angular, 

 incurved, flattened ; lateral slopes with a distinctly impressed suboval 

 space beneath the beaks, which is bounded by an angular fold extending 

 from the beak downwards to the margin of the valve at the first strong 

 plication. 



Surface marked by from thirteen to twenty-two simple strong subangular 

 plications, four to six of which are more or less elevated towards the 

 front of the dorsal valve into a mesial prominence, and three to five 

 depressed on the ventral valve so as to form a shallow sinus, and 

 produced in front into a distinct linguiform projection. 



The plications on the front of this shell have but very faint traces of the longi- 

 tudinal depressed lines so common in this type of RkynchonellcR, though occasionally 



