No. 186.] 



25 



at present known, extends from the Niagara, through the Lower Helderberg, 

 to the Hamilton ; though it is probable we shall find them in other strata. 



Under this genus I have described N. ventricosa, N. elegans and N. 

 concentrica from the Lower Helderberg rocks ; JV. pisiforniis ( Orthis 

 pzsu7n, Pal. New-York, Vol. ii, p. 250), from the Niagara group ; and JV. 

 condnna ( Atrypa concinna, Rep. of the Fourth Geol. District of New- 

 York, 1843, p. 200, f. 3), which are the species known to me at the be- 

 ginning of 1848. 



The following figures are illustrations of the structure of the genus, as 

 shown in N. ventricosa. 



Fig. 1. Interior of the dorsal^alve. 



J. Cardinal process. 

 c, c. Crural processes. 

 bp b. Dental fossets. 



r. Muscular imprints. 



5. Medio -longitudinal septum. 



Fig. 2. Profile view of the dorsal valve, showing the recurved cardinal 

 process and the bases of the spiral arms. 



Fig. 3. Interior of the ventral valve. 



2. A flattened space or false area beneath the beak. 

 t, t. Teeth. 



£. Medio -longitudinal septum. 



Fig. 4. Profile view of the ventral valve. 



Fig. 5. The interior of the dorsal valve, with a portion of the 

 ventral valve attached. The figure shows the concave or 

 grooved surface of the cardinal process j, as if for the 

 passage of a pedicle ; the bases of the crura c, c, coming 

 off at the base of the cardinal process ; while the teeth 

 of the ventral valve, t, t, fitting into the fossets below 

 these, clasp the whole firmly. 



[Assembly No. 186.] 4 



