LOWER HELDERBERG ROCKS. 260 



In the first volume of the Palaeontology of New- York, I have designated 

 by the generic name Ambontchia a group of certain species which are 

 allied in form and other external characters ; but not having at that time 

 seen the structure of the hinge, the generic description could not be 

 founded on the study of these most essential parts. Since then I have 

 obtained casts of A. radiata, and separated valves of one or two other 

 species ; and I have also seen some very fine specimens of A. orhiculata 

 and others in the collections of the Geological Survey of Canada. These 

 examples show that A. radiaia and A. cartnata have a single large and 

 nearly central muscular impression ; one specimen of the former showing 

 distinctly two strong teeth beneath the beak, while at the posterior ex- 

 tremity of the hinge-line there are three lateral elongate and slightly 

 curving teeth, the hinge-area being striated longitudinally. The cardinal 

 teeth of the A. carinata are similar, but there is some obscurity in regard 

 to the lateral teeth. These characters probably exist in all those with 

 extended and subspiral umbones and striated or costate surfaces ; but in 

 the concentrically striated forms, this hinge structure is somewhat varied, 

 and some of them at least exhibit double muscular impressions, one large 

 subcircular pit lying at the anterior extremity. 



The illustration is from a cast of this species ( the beak being 

 left out ), showing the cardinal and lateral teeth, the muscular 

 and palleal impressions. The margins of the external costse are 

 preserved in the margin of the cast. 



Ambonychia radiata. 



The A. obtusa, and an allied form from Tennessee, have the same 

 general characteristics ; while there is a distinct external ligamental 

 area, which is likewise seen- in other species. 



A farther examination has shown that some species heretofore referred 

 to the Genus Modiolopsis have this structure of hinge, muscular impres- 



