ATRYPA OF THE UPPER HELDERBERG GROUP, &c. 
319 
tral valve is depressed in a broad deep sinus, which is flattened in the bottom and quite unlike the 
preceding ones in form. This group, represented in figs. 11-13, Plate 51, and figs. 1-6 Plate 52, is 
most nearly allied to Atrypa impressa of the Schoharie grit, but the specimens do not show the flatten¬ 
ing of the middle of the dorsal valve, nor do we know that they have the extremely thickened shell, 
allowing such depth of the rostral cavity as shown in fig. 8 of Plate 51. This peculiar feature has not 
been observed ip any other of the various forms of Atrypa obtained from our rocks. 
In figs. 10 and 14-24, Plate 51, we have the representation of another group of Atrypas, differing in 
some small degree from the preceding. In these the young are but moderately convex, broader in 
form, with the striae coarser and more regularly bifurcating. So far as observed, they do not acquire 
the gibbosity of the other forms, the older shells presenting a broad undefined depression of the ven¬ 
tral valve, as shown in fig. 22, but never acquiring the character of a deep or abrupt sinus. 
These forms are known in NewYork as occurring in the higher beds of the Corniferous limestone 
near Williamsville, in Erie county, and are characteristic of the hydraulic beds at Louisville, Ken¬ 
tucky, and Jeffersonville, Indiana. 
Continuing the character and mode of development observed in the specimens as 
represented in the figures referred to, we pass to the forms designated by Vanuxem 
as A, consimilaru* (figs. 9-12, plate 52), where the front of the dorsal valve is 
often much elevated and deeply sinuate ; the ventral valve has a broad mesial 
depression, and the front produced. These are the representatives of the Euro¬ 
pean form A. desquamata. 
The varieties of form in the ventral valve are shown in figures 8, 10, 11 and 12. Figure 9 is a profile 
view of another individual, and figure 7 is a dorsal view of the same. 
The interior of the ventral valve of this form is shown in figure 51. 
In a specimen of smaller size than figures 8 and 10, and of similar proportions, we have the muscular 
and vascular impressions as shown in figure 12. There is really a considerable difference between 
the parts as shown in this specimen and in the cast of the ventral valve of A. impressa as illus¬ 
trated in figure 9, pi. 1; but there is no important difference between figures 11, 12, except in 
the proportionally greater length of the muscular impression in,the latter. 
The common forms of this species in the Hamilton group are illustrated on 
Plate liii, figures 3-11. 
In the young specimens the beak of the ventral valve is elevated and perforated at the extremity, and 
separated from the umbo of the dorsal valve, the intermediate space being occupied by a deltidium 
as shown in figures 3, 4. This feature disappears in older shells, and the perforate apex is closely 
appressed to the umbo of the dorsal valve. The hinge-line is longer and more nearly straight 
than in specimens from the Corniferous limestone. The dorsal valve does not acquire the extreme 
gibbosity, and the striae have a finer aspect. The mesial sinus usually impresses only the front of 
the shell, and is but moderately developed, consequently affecting only the front of the dorsal valve. 
* Hipparionyx [ Atrypa ] consimilaris, Vanuxem, Report on the Third Geological District of New York, 
p. 1E2, f. 2. 1843. 
