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 speciesj they are not 

 visible. 



Fig. 7 a. Dorsal valve, b. Cardinal view. e. Profile view. 



Position arui locality. This species occurs in the compact black limestone, with others 

 of the genus, at Middleville, Herkimer county. 



182. 15. ATRYPA AMBIGUA (n. sp.). 



Pl. XXXIII. Figa. 8 o, 6, c, & 9 a, 6. 



Subrhomboidal, trilobate ; length 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 b, represent this shell la its usual form, with a simple trilobate aspect, and free from 

 plications. 



Fig. 8 e. 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 

 subtriphcate 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. 



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