18 Aiiomalodonta gigantea. 



anteriorly, about J of an inch in diameter in a large specimen. Height 

 of the shell from 2 to 4 inches, greatest breadth about | less. Shell 

 (^uite thick about the umboues and wing, but thinner toward the base. 

 Large cardinal tooth or elevation beneath the umbones, and sloping 

 posteriorly from the beak. The cardinal elevation on the left valve 



Fig. 9. A. giganfea. External surface. 



having a depression to receive a corresponding elevation, though slight, 

 on the right valve. From the cardinal tooth there are from 4 to 18 

 lateral cartilage grooves extending posteriorly to the end of the wing, 

 and terminating with the shell, and there are the same number of car- 

 tilao-e grooves on the anterior side of the cardinal tooth that imme- 

 diately run together as they pass into the byssal sinus. The cartilage 

 grooves vary in number with the age and size of the shell. The shell 

 is thickened on the anterior side, and appears to show lines of growth 

 passing through the sinus to its base. A large muscular impression is 

 found near the anterior margin, half Avay from the sinus to the base of 

 the shell, and there are appearances that indicate another muscular 

 impression on the posterior wing of the shell, near its termination. 

 Greatest depth of a valve in a large specimen, \ an inch. 



This is the largest bivalve yet known in the Cincinnati Group. 

 It may readily be distinguished from the Anomalodonta data by the 

 surface markings, though the general outline form of the two shell.^ 

 are nearly the same. 



I found this species near Versailles, Indiana, about 40 miles west of 

 'Cincinnati, and about 300 feet below the Upper Silurian rocks ; and I 

 also found what I believe to be a cast of the same at Kichmond, 

 Indiana. I do not know that it can be found elsewhere, but the prob- 

 abilities are that it can be found in the upper part of the Cincinnati 

 Group, from Madison to Richmond, and at other places. 



