248 PALEONTOLOGY OF NEW YORK. 



leiij^th of tlio two valves respectively is 63 and 43 mm. In anotlier 

 .specimen (plate xxvii, figs. 6 and 7), where the two valves are in juxtapo- 

 .^ition, there appears to be little difference in the relative dimensions, but 

 this appearance is in a great measure due to the fact that the margins of the 

 left valve have been l)roken away to tlio line nl' imiction of iIil' two valves. 

 Tlie proportional (liiiicnsions of the left valves are subject to iimch variation, 

 as is shown in ])late xxvii, figs. 2 and 3. 



The measurement of two large specimens gives respectively, length 78 

 and 74 mm., height '.•() mm. each, and hinge-line 58 and 65 mm. A large 

 specimen with the margin imperfect has a length of 88 mm., and height 112 

 mm. A separated right valve has a length of 97 mm., heiglit 103 mm., and 

 hin2:e-line 84 mm. 



The young shells ol' this form, especially the right valves, might readily be 

 mistaken for Aviculopecten or Pterinopeden ; but a critical examination will show 

 the radii to be peculiar and chiiracteristic of the genus. 



The form and proportions of the fossil are subject to great \ariations in 

 their natural conditions, but the variation and distortions of form, consequent 

 upon the degree and direction of pressure from the inclosing matrix, produces 

 much greater extrcMiics. Some of the distorted forms are illustrated on plates 

 xxviii and xxix. In llie older specimens, the obliteration or obsolescence of 

 the radii beloAv the middle of the valve, gives a great variety of aspect to the 

 fossil. These variations are illustrated on plates xxvi, xxvii and xxviii. 



In all the younger specimens, the wing is produced, ami the margin sinuate, 

 while the surface is distinctly marked by the radii. These features are 

 gradually lost as tlie slu-ll advanct-s in age, until the margin of the wing 

 becomes nearly oi- (|nit(' direct, the extremity of the wing obtuse, and tlie 

 surface marked only by the concentric strise of growth. In old specimens the 

 ear is often obscure, and scarcely retains the pro])ortionate size of that 

 appendage in the younger shells. 



Formation and localities. In the Hamilton group, at Cazeuovia and other 

 localities in Madison county, and in Otsego and Onondaga counties, N. Y. 



