26 



[Assembly 



Fig. 6. The ventral valve, showing the spires as detached from 

 the dorsal valve, and the arching transverse filament 

 which connects the two a little below their base. 



Fig. 7. A longitudinal section of the two valves in their natural 

 relations te each other, showing the cardinal process j 

 of the dorsal valve extending beneath the false area of 

 the opposite valve, and curving into the rostral cavity. 

 The crura c, from which the spiral arm on one side is 

 ^ shown, first bend a little into the ventral cavity, and then 



turn abruptly backwards and curve into the dorsal ca- 

 vity, following essentially the curvature of the valve : 

 about midway between the beak and base is shown one side of the connecting process 

 I, which tends abruptly upwards and backwards, terminating in an acute point which 

 is directed towards the free extremity of the descending branch of the crural process. 

 The medio-longitudinal septa s, s, are of equal elevation in the two valves. 



Fig. 8. Dorsal valve, showing the attachment and 

 base of the crura. 



These figures are all enlarged three diameters, excerpt fig. 8, which is 

 four diameters. Every part here shown has been clearly seen in specimens 

 of N. ventricosa, and the structure is corroborated by specimens of JV. 

 concinna. 



Fig. 9. Dorsal and cardinal views of a specimen of Nuchospira 

 concinna (Mrypa concinna), from the Hamilton group 

 of New-York. 



For farther illustrations of the genus and species, see Vol. iii, Palaeonto- 

 logy of New- York, Plate xviii b. 



