PLATE LI. 



Ctrtoceras citum. 



PaRC 372. 

 Tig. 1. Lateral view of a small fragment showing- the curvature of the tube, and the reg-iilar numerous 



annulations of the internal mould corresponding to the transverse ridges of the test. 

 Fijr. 2. Ventral view of the preceding, showing the characters of the sinus in the lamellae. 

 "Fig. 3. Longitudinal section of a fragment of another individual, showing the curvature and enlargement of 

 the tube, but preserving no traces of the septa. 



The specimens of this species here illustrated are from the Upper Helderberg limestone, at 

 Schoharie, N. T. 



Gtroceras Nereus. 



Page 373. 



Fig. 4. Lateral view of an individual, showing the depth of the air-chambers and the curvature of the tube. 



Fig. 5. The opposite view of the preceding which pi-eserves, on this side, portions of the test with its orna- 

 ments. 



Fig. 6. A portion of the surface enlarged, showing the plicate ridges and the fine, undulating lines of growth. 

 Upper Helderberg limestone. Auburn, iV. Y. 



