10 



PAL^ONTOLOGV OF N£W-VORK. 



39G. 23. ATRYPA PLICATA ( n. sp.) 



Fl. IV. Fig. 6. 



Smaller valve stiborbicular, wider than long ; surface marked by 12 sharp plications, withotfl 

 any perceptible mesial elevation ; plications marked, in the shell, by fine concentric lines ; beak 

 small, incurved. 



The specimen is the cast of a single smaller valve, but it presents characters unlike any of 

 the preceding species. In its form and plications it resembles the A. increbescens , but differs iii 

 having no mesial elevation, and the form of the cast at the beak (which is not well represented 

 in the figure) differs essentially from that shell. 



Position and locality. This shell occurs with the following species, in the higher red portions 

 of the rock at Lockport. (.Collection of Col. Jewett.) 



391. 17. MODIOLOPSIS ORTHONOTA. 



Pl. IV. A. Figs. 1 a, b, c. 

 Unio mthonotus. Conrad, Annual Report of 1839, p. 66. 

 Cypricardia orthonota. Hall, Geol. Report 4th Dist. New-York, 1843, pag. 48, figs. 8 & 9'. 



Subelliptical or rhomboidal, obliquely truncated before ; hinge-line straight ; posterior 

 extremity rounded or obliquely truncated ; sides nearly parallel ; umbones elevated, thin, sharp, 

 with a defined ridge extending, to the posterior basal margin ; anterior to the ridge is a flattened 

 space widening from the umbones towards the base, anterior to which the shell is a little more 

 ventricose ; muscular impression at the anterior extremity distinct,, no other visible j surface 

 marked by fine concentric lines. 



The specimens of this species are usually casts, preserving some faint concentric lines, the 

 markings of the shell. There is little difficulty in identifying this species by its general form. 

 The characters are not always so clearly defined as in the larger specimen figured ; but in all 

 the variety of smaller forms, there is not a sufficient departure from those figured as to mislead. 

 Some of the smaller specimens bear a close resemblance to M.faba (^Paleontology, Vol. i. 

 pag. 158, pl. XXXV. fig. 6 a, b, c) ; but the hinge-line is much more straight, and the posterior 

 extremity usually less rounded ; and the sinus, which is so conspicuous in M.faba, is only 

 represented by a flattened or slightly depressed space. 



This species is quite abundant in some of the higher layers of the rock. 



Fig. 1 a. Eight valve of a small specimen, with the posterior extremity rounded and the anterior 



one extended. 

 Fig. 1 b. Right valve of a larger specimen,- having the anterior extreihity less prolonged. 

 Fig. 1 c. View of the hinge-line and cardinal area of the last. 



Position and locality. This shell is usually found about fifty or sixty feet below the top of 

 the sandstone at Medina, and in the higher part of the rock at Lockport. {State Collection.) 



