38 Bulletin 14 304 



fold extending from the front about one-third the length of the 

 shell. The sinus of the ventral valve is shallow, equal in length 

 to the fold. 



Shell thin, the surface marked by fine concentric strise which 

 undulate in crossing the fold and .sinus. The surface of the cast 

 is marked by vesy numerous fine, wavy, radiating lines. 



This small species differs from Triplecia nucleus Hall in the 

 presence of radiating markings, less convexity of valves, and in 

 the shape and extent of the fold and sinus. It differs from the 

 young of Triplecia extans Emmons in the shape and extent of the 

 fold and sinus and in the length of the cardinal line. 



Locality. — Found in zones A 8, B 7, 11, 13. 15, and C 12 of 

 the Chazy limestone, Crown Point, N. Y. Also at Valcour 

 Island, N. Y. 



Triplecia extans Emmons, PI. 19, fig. 4 



This figure is introduced for comparison with Triplecia 

 gracilis of the Chazy. It was drawn from a rather large speci- 

 men collected in zone C 27 of the Black River limestone. 



PLECTORTHIS Hall and Clarke 



Piectorthis plicatella Hall, PI. 19, figs. 5, 6 



Hall's description: "Broadly semioval, nearly aequivalve, 

 length and breadth about 3 to 4. Surface marked by strong, 

 radiating plicse, which are' usually simple, about 20 to 28 on each 

 valve, crossed by simple elevated concentric lines, which are more 

 distindl in the depressions between the costse, and often obscure 

 or obsolete upon their exposed surfaces; valves nearly equally 

 convex without sensible depression or elevation on either one, 

 meeting at the edges in a straight line; cardinal line not extend- 

 ing beyond the width of the shell; area narrow; dorsal foramen 

 extending to the beak." 



In the cast of the pedicle valve the muscular scars are sub- 

 elliptical and small, but strongly impressed. 



Locality. — Rather common in zones C 19, 20, 23, 24 of the 

 Black River limestone, Crown Point, N. Y. 



