328 JRemarlcs upon the Genus Anomalodonta, etc. 



an Amhonychia radiata. Shell marked, exteriorly, toward the margin, 

 with lines of growth and concentric striaj, which cross the radii, ren- 

 dering it likely that concentric stride crossed the radii over the whole 

 snrface of the shell, though they appear now to be smooth." 



2. Alata. — " kSurface ornamented by about 24 to 28 simple, strong, 

 radiating costse to each valve, that are nearly equal in breadth to the 

 furrows between ; those on the central portions of the valves passing 

 nearly straight from the beaks obliquely to the posterior basal margins, 

 those on the anterior side curving more or less forward below, and 

 those near the cardinal margin curving a little upward behind ; in 

 some examples, crossing all of these costse and the furrows between, 

 are numerous, fine, crowded lines, and at regular, distant intervals a 

 few strongly defined, imbricating marks of growth, that curve parallel 

 to the basal and j)osterior margins." 



Now, let us see what are the specific differences here manifest to an 

 ordinary paleontologist : 



1. The glgantea has 80 to 40 strong radii, while the alata has 24 to 

 28. Let it be borne in mind, that the chief specific differences between 

 the three species of Amhonycliia (radiata, hellistriata, and costata}, are 

 founded upon as little variation in the number of the radii, or costse. 



2. The radii on the gigantea are the same width as the intermediate 

 spaces, and have much the same appearance as those on an Amhonycliia 

 radiata, while the radii on the alata are nearly equal in breadth to the 

 furrows between, but those on the central portions of the valves pass 

 nearly straight from the beaks obliquely to the posterior basal margin, 

 while those on the anterior side curve forward below, and those near 

 the cardinal margin curve upward behind. Here is a very important 

 distinction, for while the radii curve on the alata, the spaces between 

 them grow wider and wider, but they increase very little in width, if 

 at all, as plainly appears by the figure of Prof. Meek, referred to 

 above ; thus ditfering widely from the Amhonycliia radiata and the 

 gigantea, as shown by the description and figure above referred to. 

 Indeed, no other shell of the same, or an allied genus, found in the 

 Cincinnati Group, possesses this peculiarity in the same degree as the 

 aJata. 



3. The gigantea is marked exteriorly, toward the margin, with lines 

 (jf growth, while the alata has at regular distant intervals a fev/ 

 strongly defined, imbricating marks of growth, that curve parallel to 

 the basal and posterior margins. There is no resemblance at all 

 between the two species in this regard, and this ought to be apparent 

 to any one, as well from the descriptions as from a view of the engrav- 

 ings ; but, for the purpose of enlightening the Professor upon this speci- 



