Genus AmbonycMa. 15 



"Obliquely subovoid, ventricose, very inequilateral, height much 

 greater than the length ; umbones very ventricose, and extended into 

 long and incurved beak;^, which bend forward at their extremities ; an- 

 terior margin not alated, or extending beyond the beaks ; posterior 

 side compressed, subalute ; base regularly rounded ; cardinal line short, 

 very oblique to the umbones ; surface marked by fine radiating stride, 

 with a few elevated concentric lines of growth." 



" Tills beautiful fossil shell is readily distinguished by its external 

 form and markings. The umbones are extended into long incurved 

 beaks, which are very ventricose, and rise abruptly from the shell to- 

 Avard the summit ; the central and lower part of the shell is regularly 

 convex, becoming more compressed toward the margin. The anterior 

 side extends almost in a right line from the beaks toward the base." 

 [Pal. of K Y., vol. i., p. 163.] 



I have the hinge line of this species from the excavation for Colum- 

 bia avenue, at an elevation of about 150 feet above low water-mark. 

 It exactly agrees with figure 5 above. 



The form most common about the stone quarries at tlie top of the 

 hills back of Cincinnati has been by Prof. Hall himself referred to his 

 species from the Hudson river group, A. radiata. This is the most 

 common AmbonycMa in the Cincinnati group, though it is very rare 

 to find anything more than a mere cast of the fossil. It prevails 

 higher up m the rocks quite as abundantly as it does at Cincmnati, 

 and in a condition quite as unsatisfactory and imperfect. It is de- 

 scribed as follows : 



" Equivalve, obliquely obovate, extending into acute curving beaks ; 

 anterior slope nearly straight above and rounded below ; posterior slope 

 obli(pie, scarcely alate ; surface marked by twenty -five to forty strong, 

 simple radii, which are crossed by fine, concentric strite ; radii flat. 

 tened upon the top; the intermediate spaces are regulaiiy concave 

 grooves, narrower than the radii, and marked by the concentric stride." 

 [Pal. of K Y., vol. i., p. 292.] 



Another form, the A. Costata, is described in the "Paleontology of 

 Ohio," vol. i, p. 130, from a point 350 feet above low water-mark, at 

 Cmcinnati. It is distinguished from the A. radiata " by its small num- 

 ber and more widely sejiarated costa;, and rather compressed, narrow 

 form," and " in having its costre separated by flat interspaces, instead 

 of regularly concave grooves, narrowei- than the radii." 



These are all the species that have been clearly identified in the 

 Cincinnati Group, that belong to this genus, unless it should be the 

 A. Casei, which I believe to belong to another genus (Anovialodonfa). 

 There are, however, other species, unless the A. radiata had many var- 



