NOTICE 



OF THE 



AMBONYCHIA, PAIAEARCA Affl 



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

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

 allied in forfti and other external characters ; hut not having at that time 

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

 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 in the collections 

 of the G-eological 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 extremity 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 

 unbones 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 marks 

 f the costae are preserved in the margin of the cast. 



Ambonychia radiata 



