CRYPTONELLA OF THE HAMILTON GROUP. 395 



Geological formation and localities. This species occurs in the Hamilton group, 

 at Bellona in Yates county; on the shore of Canandaigua lake ; at York, Moscow 

 and Genesee in Livingston county ; at Hamburgh on the shore of Lake Erie, and 

 elsewhere in Western New- York. ' 



Crjptonella plaiiirostra. 



PLATE LXI. 



Terebratula planirottrit : Hall, Thirteenth Report on the State Cabinet, p. 89. 1860. 



" " Id. Referred to Cbtptonella in Fourteenth Report, p. 101. 1861. 



Cryptonella planirottra i Id. Sixteenth Report on the State Cabinet, p. 44. 1863. 



Shell subovate or elliptical ; the front often truncate, giving a subpenta- 

 gonal outline ; the greatest width near the middle of the shell; the 

 length and width about as six to seven or eight. The depth of the two 

 valves, in well-formed specimens, is about equal to half the length. 

 Ventral valve in old shells gibbous, the greatest convexity above the 

 middle, often slightly flattened towards the beak, which is moderately 

 incurved and truncate by a round foramen, limited below by conspi- 

 cuous deltidial pieces, abruptly inflected along the umbonal slope, 

 often leaving an angular ridge, and the space between it and the hinge- 

 margin flat or concave ; often slightly flattened below the middle, and 

 regularly curving to the sides. 

 Dorsal valve considerably shorter than the opposite, more convex or 

 gibbous, the greatest convexity at or a little above the centre, curving 

 to the sides and front where it is sometimes flattened : beak incur- 

 ving beneath the deltidial plates of the opposite valve. 

 Surface marked by fine rounded concentric striae, which in old shells 

 are disposed in broader belts of imbricating lamellae. Shell-structure 

 punctate. 



The crura reach below the middle of the length of the shell, 

 approaching each other very closely, and abruptly recurved for a dis- 

 tance about half their length : they are united near their bases by a 

 transverse band. 



