248 PALEONTOLOGY OF NEW YORK. 



Length of the two valves respectively is 63 and 43 mm. In another 

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

 sition, 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 broken away to the line of junction of the two valves. 

 The proportional dimensions of the left valves are subject to much variation, 

 as is shown in plate xxvii, tigs. 2 and 3. 



The measurement of two large specimens gives respectively, length 78 

 and 74 mm., height 90 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., height 103 mm., and 

 hinge-line 84 mm. 



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

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

 the radii to be peculiar and characteristic of the genus. 



The form and proportions of the fossil are suhject to great variations 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 extremes. Some of the distorted forms are illustrated on plates 

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

 the radii below 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, and the margin sinuate, 

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

 gradually lost as the shell advances in age, until the margin of the wing 

 becomes nearly or quite direct, the extremity of the wing obtuse, and the . 

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

 ear is often obscure, and scarcely retains the proportionate size of that 

 appendage in the younger shells. 



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

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



