EXPLANATIONS OF PLATE I. 



[This Plate, with the paper on the Internal Appendages of the Genus Atrypa, was com- 

 municated to the Nineteenth Report on the State Cabinet, and although printed 

 and published in 1866 was not included in that Report, In making up the Twentieth 

 Report this plate was overlooked, and there are consequently duplicates of Plate I.] 



Fig. 1. View of a specimen of Alrypa reticularis from the Lower Helderberg group 

 of Albany county, N. Y., showing the form and position of the loop and 

 spires. The view is obliquely from above and behind. Enlarged. 



'' 2. Interior of a ventral valve •, from the same position and locality as the last, 

 enlarged to show the depressed area and solid beak. 



" 3. Interior of a ventral valve of A. reticularis from the Hamilton shales, show- 

 ing the form of the beak as existing in very perfect specimens ; for com- 

 parison with fig. 2. The specimen is in Prof. Hall's collection, and the figure 

 borrowed from the Pal. of N. Y., Vol. IV. 



^' 4. An enlarged outline figure of the first volutions of the spires with the loop and 

 crura, attached to the hinge-plate. The specimen from which the figure 

 was drawn is from the falls of Ohio, and belongs to the collection of Prof. 

 Hall, from whom it was obtained for manipulation. 



^*' 5 & 6. Dorsal and ventral views of a specimen from the shales of the Hamilton 

 group of New-York, showing the form of spires and loop. 



'' 7 & 8. Dorsal and ventral views of a specimen of Alrypa spinosa, Hall, from 

 the Hamilton shales of New- York, showing the form of spires and loop. 

 Figs. 4 — 8 are natural size. 



