LOWER HELDERBERG ROCKS. 
269 
In the first volume of the Palaeontology of New-York, I have designated 
by the generic name Ambonyciiia 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. orbiculata 
and others in the collections of the Geological Survey of Canada. These 
examples show that A. radiata and A. carinata 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 cost* are 
preserved in the margin of the cast. 
Ambonyciiia 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- 
