LAMELLIBRANCHIATA. 163 



strongly marked on the cardinal expansions and over tlie hinge-margin. 

 In well-preserved specimens the undulations are stronger upon the middle 

 of the valve, and less marked upon the anterior side and the wing. In the 

 right valve the}* are much more regular and subdued. 



Interior characters unknown. Ligamental area marked by a distinct 

 groove parallel to the margin. 



The largest specimen has a length of oO mm., height 40 mm., hinge-line 

 about 45 mm. A small specimen measures 26 mm. in length, 39 mm. in height, 

 hinge-line imperfect. Another specimen is 26 mm. in length, 33 mm. in 

 height, hinge-line 27 mm. A right valve is 33 mm. in length, height about 

 the same, hinge-line 41 mm. 



This form resenrbles, in general surface characters, the three preceding 

 species, but differs by the more erect body and sul^-orbicular form. In general 

 aspect it resembles Glyptodesma, and the surtace markings are not very differ- 

 ent, though none of the specimens show the hinge characters of that genus. 



The description has l)een drawn from specimens preserved in line calcareous 

 shale and in coarser arenaceous material, and the characters remain essen- 

 tially the same in both. 



In a single specimen from the arenaceous shales on the shore of Cayuga 

 lake, at Norton's Landing, where the shell is partially exfoliated, the surface 

 is marked by fine, concentric lines of growth, and more distant lamellose stria", 

 which are about twice as numerous as in the typical specimens. These give a 

 gently undulating chai'acter to the surface of the cast of the left valve, instead 

 of the abrupt angular folds of the typical specimens. In the right valve of the 

 same specimen the striae on the umbo ;ire similar, but more closely arranged, 

 and on the lower portion, in the cast, they exliibit the gentle undulations of the 

 typical forms of the right valve. 



Formation and localities. In shales of the Hamilton group, in Ontario county ; 

 Norton's Landing, Cayuga county ; Bellona, Yates county, and in the coarse 

 grits at Leonardsville, Madison county, N. Y. 



