14 



Genus Ambonychia. 



Genus Amhonyehia (Hull), 1847. [ral. of N. Y., vol. i, p. 1G3.] 

 [Greek ambou, the boss of a shield, and onijx, a claw or talon ; in 

 allusion to the rounded, incurved umbones in the typical species.] 



" Equivalve, inequilateral, compressed, alate or subulate posteriorly, 

 obtuse and abruptly declining, or curving downward on the anterior 

 jnargin. General form somewhat obliquely ovate, gibbous or inflated 

 toward tlie umbones and on the center of tlie shell ; cardinal margin 

 very oblique, or approaching a line parallel to the direction of the um- 

 bones, which are often incurved at the extremity, and equal or project 

 beyond the line of the anterior extremity ; surface marked by more or 

 less prominent concentric strise — strong undulations or fine radiating 

 strire. Muscular impressions large : one in each valve."' 



[Note to the genus Ambonychia, with cuts from "Pal. ofN. Y.," vol. 

 3, p. 2G9-523.] 



" Figure 5 shows the hinge line, tlie cardinal teeth f, and the lateral 

 teeth tt. On the anterior side the mai-gin of the shell is sinuate, for 

 the passage of the byssus h. The latter character is likewise more dis- 

 tinctly shown in figure 6, which is an anterior view of the right valve.'' 



l-iK. 



Fig. G. 



" There are two strong teeth beneath the beak, while at the posterior 

 extremity of the hinge line there are three lateral, elongate and slight- 

 ly curving teeth, the hinge area being striated longitudinally." Tlie 

 A. radiata has a single large and nearly central muscular impression. 



Fossils of the genus Ambonychia are found throughout the Cincin- 

 nati group, from low water-mark at Cincinnati, to the Upper Silurian 

 in Ohio, and at Versailles and Richmond, Indiana. Whether any one 

 species extends through this great depth or not is at least extremely 

 doubtful. The form most common at Cincinnati, within two hundred 

 feet of low w^ater-mark, appears to agree with Hull's Trenton limestone 

 species, A. bdllitriata, which he described as follows : 



