INVERTEBRATES. 307 



size, deeply embracing, abruptly rounded, or subangular around 

 the umbilicus; septa not very deeply concave, arching slightly 

 forward in crossing over the dorsum, where they are separated 

 by spaces a little less than one-fourth their own transverse 

 diameter ; siphuncle very near or quite central; aperture trans- 

 versely reniform, being modified by the inner whorls. (Sur- 

 face unknown.) Length or greatest diameter, about 2.46 

 inches; height, 2.05 inches; breadth near the aperture, 1.70 

 inches; breadth of umbilicus (in cast), about 0.5.0 inch. 



At a hasty glance this and the last described species might be confounded 

 by a careless observer; but it will be seen by comparison, that the form now 

 before us is much more compressed, and has a smaller umbilicus ; while its 

 whorls are proportionally deeper in the direction of the greater diameter of the 

 shell, and not near so angular around the umbilicus on each side. 



Locality and position : Same as last. 



Subgenus ENDOLOBUS, M. and W. 



(ev<$ov, within; Xofioc,, a lobe.) 



(Proceed. Acad. Nat. Sci., Philad., Dec, 1865, p. 259.) 



Shell compressed or subdiscoidal; umbilicus wide and show- 

 ing all the inner turns; volutions more or less rounded, and 

 without longitudinal furrows or ridges, but slightly embracing ; 

 septa bent backward on the inner side, so as to form a kind of 

 funnel-shaped ventral lobe, like we see in Tretoceras, excepting 

 that it is not so deep. Siphuncle central, or between the mid- 

 dle and the outer side. 



The type of this subgenus differs from other large palaeozoic NautiKi, with a 

 wide, open umbilicus and merely contiguous or moderately embracing whorls, 

 in having the septa on the inner side bent backwards, so as to form a shallow 

 kind of inner or ventral lobe. It is probable that Montfort's name Bisiphites, 

 which was proposed for a large fossil shell, apparently of Carboniferous age, 

 may be synonymous with our Undolobus, as he described his type as having two 

 siphons, one nearly central and the other near the inner side, while it would be 



