ATRYPA OF THE UPPER HELDEEBERG 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 in any other of the various forms of Atetpa obtained from our rocks. 



In figs. 10 and 14-24, Plate 51, we have the representation of another group of Atrypaj, difieringin 

 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. 2Z, but never acquiring the character of a deep or abrupt sinus. 



These forms are known in New York as occurring in the higher beds of the Corniferous limestone 

 near William-sville, in Erie county, and are characteristic of the hydraulic beds at Louisville, Ken- 

 tucky, and Jeflcrsonville, Indiana. 



Contiuuiiig 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 

 &a A. const milaris* (tigs. 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 s 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 difierence between 

 the parts as shown in this specimen and in the cast of the ventral valve of ^. impressa as illus- 

 trateii 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 striie 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 iAtrypa] coiuimilarU, Vanoxem, Report on the Third Geological District of New York, 

 p. 132, f. 2. 1843. 



