ORTHIDES OF THE PORTAGE AND CHEMUNG GROUPS. ' 59 



In many of its characters this species assimilates with the following, and it is 

 possible that intermediate forms may connect them, though I have not at present 

 the means of making this comparison fully. 



Geological formation and locality. This species occurs in some beds of brown 

 sandstone near Paintedpost, New-York, and has not been obtained in any other 

 locality. 



Ortliis tioga ( n. s.). 



PLATE VIII. 

 Referred to OrthU interlineata, Sowebbt, in Report on the Fourth Geological; District New-Tork, p. 263. 



Shell transverse, broadly elliptical, about two-thirds as long as wide : 

 length of hinge-line a little greater than half the width of the shell ; 

 the. extremities rounded into ff general curved outline. Dorsal valve 

 convex, the greatest elevation near or above the middle on each side 

 *of a well-marked mesial sinus ; sometimes a little flattened at the sides, 

 and regularly curving to the front. Ventral valve very gently convex, 

 with sometimes a slight mesial elevation : area of medium width, with 

 the beak slightly incurved ; foramen wide. 

 Surface striated : striae angular, often fasciculate, curving upwards on 

 the hinge-line.  



The shell is rarely preserved ; but when it is seen, the striae are an- 

 gular, and every second one in the upper part of the shell, and every 

 third or "fourth one in the lower part of the shell, are stronger and more 

 prominent : concentric striae fine and closely arranged. In the casts, the 

 character of the striae is tolerably well preserved in the fasciculate ar- 

 rangement, and in the curving upward at the sides and on the hinge-line ; 

 while on the margin, they are much more strongly impressed than on the 

 middle of the cast. When the cast is preserved in fine shale or shaly 

 sandstone, the surface is minutely punctate ; and the shell, when pre- 

 'served, -has the same character. 



The muscular impression in the dorsal valve is usually but faintly 

 defined in the casts, which preserve the impression of a strong cardinal 

 process, bifid at the extremity. The socket plates are strong and widely 

 diverging, and the area is extremely narrow. In the cast of the ventral 



