NUCLEOSPIRA OF THE HAMILTON GROUP. 279 



side of the valve often extends from beak to base. Dorsal valve 

 furnished with a strong spatulate cardinal process, which, rising verti- 

 cally from the cardinal margin, is closely grasped at its base by the car- 

 dinal teeth of the other valve ; and thence bending abruptly upwards, 

 and expanding, is projected into the cavity of the opposite beak, lying 

 close upon the underside of the false area. This process is grooved or 

 depressed in the centre of the upper side, so as to leave between it and 

 the arch of the ventral beak a narrow space for the passage of a pedicle, 

 a minute foramen being sometimes observed in the beak for its pro- 

 trusion. From the sides of this process, above the junction of the teeth 

 of the opposite valve, and at the point where it bends upwards, originate 

 the crural processes which support the spires. A deep cavity beneath 

 the cardinal process extends to the dorsal beak, where originates a thin 

 elevated septum often extending to the base of the shell. Muscular 

 impressions of the dorsal valve occupy a narrow oval space ; those of 

 the ventral valve broader, flabelliform. 



Under this genus I have described N. ventricosa, N. elegans and N. concentrir.a from the 

 Lower Ilelderbcrg rocks; N. pisiformis (Orthis pisum. Pal. New- York, Vol. ii, p. 450), 

 from the Niagara group ; and N. concinna ( Alrypa concinna. Rep. of the Fourth Geol. Dis- 

 trict of New-York, 1843, p. 200, f. 3), which are the species known to me at the beginning 

 of 1858. 



iVucleospira concinna. 



PLATE XIV. 



Mrypa concinna : Hall, Geological Report Fourth District New-Tork, p. 200, f. 3. 1843. 

 Ifucteotpira concinna .- Hall, Twelfth Report on the State Cabinet, p. 25 and p. 26. 1859. 



Shell depressed, subspheroidal, nearly circular in outline, the width 

 being usually' a little greater than the length ; valves subequal. 



Ventral valve regularly convex, the greatest convexity a little above 

 the middle, and curving regularly to the sides and front ; umbo pro- 

 minent, the beak neatly pointed and incurved over the apex of the 

 dorsal valve, leaving a space between which sometimes exposes the 

 narrow area. There is usually a narrow depressed line from the beak 

 to the base of the valve ; but this is sometimes partially absent, or so 

 faint as not to be readily observed. 



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