308 PALAEONTOLOGY OF ILLINOIS. 



very easy for such an observer to mistake the lobe we have mentioned for a second 

 siphon, when filled with strony matter. At any rate, that he was mistaken in 

 regard to the existence of two siphons none will doubt. Even in case his type 

 has, however, the internal structure of our shell, we think his name should not 

 be retained, as it implies a direct contradiction of fact. 



Nautilus (Endolobus) speCtabilis, M. and W. 



PI. 25, fig. la, 16. 



Nautilus speclabilis, Meek and Worthen, October, 1860. Proceedings Acad. Nat. Sci., 



Philad., p. 469. 

 Nautilus (Endolobus) per amplus, Meek and Worthen, December, 1865. Ibid., p. 259. 



Shell attaining a very large size, compressed subglobose; 

 umbilicus rather deep, about as wide as the dorso-ventral 

 diameter of the outer volution, and showing about three-fourths 

 of each inner turn. Whorls three to three and a half, increas- 

 ing rather rapidly in size, broadly rounded over the dorsal or 

 outer side, and more narrowly round on each lateral margin, 

 where the greatest prominence is a little within the middle ; 

 lateral margins each provided with a row of large, broad, de- 

 pressed or undefined subnodose prominences, about fourteen of 

 which may be counted on each side of the last turn ; from these 

 rows of nodes or mammillary protuberances, the inner side of 

 each whorl rounds abruptly into the umbilicus, and is provided 

 along the middle with a moderately deep, rounded concavity 

 for the reception of each succeeding inner turn. Septa deeply 

 concave on the side facing the aperture ; separated by intervals 

 measuring on the dorsal side, more than one-third the dorso- 

 ventral diameter of the whorls at the point of measurement, 

 passing nearly straight over the broad periphery, and with a 

 very slight forward curve across the sides ; while on the lobed 

 inner concave side they are rather shallow, and more or 

 less funnel-shaped. Siphon placed rather more than its own 

 breadth nearer the inner than outer side. Surface of cast 

 smooth. Greatest diameter, 23 inches ; transverse diameter, 



