FOSSILS OF THE NIAGARA GROUP. 387 



of Leptodomus of McCoy ; but it cannot nevertheless be referred pro- 

 perly to that genus. 



The general form of the shell is subrhomboidal, with elevated beaks. 

 The casts present evidence of a large triangular cartilage pit beneath the 

 beaks ; and just anterior to this, and separated by a thin process on each 

 valve, is an apparent second pit. No teeth have been discovered on the 

 extension of the hinge-line. The muscular impressions are faint and 

 the shell thin. 



It is possible that there may have been a cartilage pit and adjacent 

 tooth, as in Mactra and Amphidisma — a feature Avhich cannot be satis- 

 factorily determined except from an examination of better specimens 

 than we possess. There is a flattened external ligamental area not unlike 

 that of the Arcacea. 



Amphiccelia leidti, n. s. 



PLATE XIV, PIGS. 13, 14, 15. 



Ambonychia neglecta (J), McChesney. New Palaszoic Fossils, p. 88. 1861. 



Amphiccelia neglecta (.'') McChesnet. Trans. Chicago Acad. Sci., I, p. 41, Plate ix, fig. 2. 1867. 



Shell equi valve, inequilateral, somewhat rhomboidal, gibbous except 

 upon the expanded posterior side ; height and width subequal ; 

 umbones gibbous ; beaks much elevated and incurved, pointed, fall- 

 ing from one-fourth to one-third the width within the anterior 

 margin, which declines from the hinge-line at a very obtuse angle ; 

 hinge-line equaling somewhat more than half the width of the 

 shell. 



The casts show a large triangular pit beneath the beak, and sometimes 

 there is evidence of a thin dividing septum. There are no visible lateral 

 teeth. The surface of the casts is usually smooth, or showing only a few 

 strong lines of growth. In a single specimen preserving a portion of 

 the shell, the surface is marked by fine close radiating strise. 



The height of the shell from beak to base measures in different speci- 

 mens from two to two and a quarter inches, with a width almost precisely 

 corresponding. The depth of the two valves is about one inch and five- 

 eighths. Some smaller specimens, which may be of this species, have a 

 length and breadth of half these measurements. 



Owing to pressure and other causes, the species exhibits great variation 

 in form and proportions. Among the specimens are two with less 

 elevated and more approximate beaks, and less gibbous form, with a 



