TRENTON LIMESTONE. 143 
This shell appears sufficiently distinct from any other species in this rock, to merit a 
separation. Its circular form, and equally convex valves with margins almost uniform, 
render it difficult to imagine a gradation to the succeeding species. It must be confessed, 
however, that it approaches in some degree to the young of fig. 10; though in fig. 10 d, a 
specimen of equal size, the plications are very distinct, while, in this species, they are not 
visible. 
Fig. 7 a. Dorsal valve. 6, Cardinal view. ¢. Profile view. 
Position and locality. This species occurs in the compact black limestone, with others 
of the genus, at Middleville, Herkimer county. 
182. 15. ATRYPA AMBIGUA (n. sp.). 
Pu. XXXII. Figs. 8 a,b,c, & 9a, bd. 
Subrhomboidal, trilobate; lengih and breadth about equal; cardinal line somewhat 
extended in a regular curve ; beaks subequal, approximate ; dorsal valve convex near the 
beak, depressed towards the sides, with a deep, somewhat flat mesial sinus, which is 
extended and considerably elevated in front; ventral valve with a prominent elevated 
mesial fold along the middle, which disappears before reaching the beak ; surface crossed 
by fine concentric lines, with a few obscure or incipient plications in the mesial fold and 
sinus ; sides of the shell rarely plicated. 
Figs. 8 a and 4, represent this shell in its usual form, with a simple trilobate aspect, and free from 
plications, 
Fig. 8 c. Front view in outline, showing two incipient plications in the sinus. 
Fig. 9. Four valves are represented precisely as they occur on the surface of a slab of limestone. In 
two of these figures, a ventral and dorsal valve, the shell is free from plications either on 
the mesial fold or on the sides ; while in a (a dorsal valve), both the mesial sinus and the 
sides of the shell are very distinctly plicated, and the contiguous figure of a ventral valve is 
subtriplicate in the mesial fold only. 
This great deviation in external markings suggests the inquiry whether this shell may 
be a variety of some other species. The simple unplicated form approaches to Atrypa 
nucleus ; but the form of the shell, the mesial sinus, and its extension in front, are all too 
different to allow of our confounding them. On the other hand, the plicated forms bear 
some resemblance to fig. 10, from which this differs in the greater length of the sinus, 
which extends nearly to the beak, while in that one it is short, terminating abruptly, and 
extending little more than half way to the beak ; and in young shells, where the incipient 
plications are observed, the sinus is barely perceptible. 
Position and locality. In the lower part of the Trenton limestone at Middleville. 
