CKETAOEOUS FOSSILS. 445 



strongly involute, the outer embracing the inner so as to leave only a min- 

 ute umbilical opening, and in some cases the umbilicus is entirely closed, 

 concealing the inner coils. Outer volution large and deflected in nearly a 

 straight line for some distance, then recurved and extending nearly to the 

 center of the straightened portion. Aperture considerably contracted and 

 hood-shaped. Lateral diameter of the outer volution and of the deflected 

 portion somewhat exceeding the dorso- ventral diameter ; the form of a 

 transverse section being equal to the larger two-thirds of a broad ovate 

 figure. The' chamber of habitation in the large individual figured on Plate 

 1 3, Figs. 2 and 3, extends to about the middle of the straight portion of the 

 outer volution. 



Surface of the shell marked by closely arranged, subangular costse, 

 which are even and of uniform size on the back and to near the middle of 

 the sides, where from four to seven of them unite in forming broad, rounded 

 folds, which contract in width on approaching the umbilicus and become 

 prominent and angular. On the involute parts of the shell the costse are 

 directed nearly straight across the volution, but on the deflected portion 

 they have a slightly forward curvature in passing from the ventral to the 

 dorsal portion, and the costal folds are stronger and more distant. 



Septa rather simple in their structure and closely arranged. The 

 siphonal lobe is about as wide as long, and provided with two bifurcating 

 branches on each side, the inner fork of each being longer than the outer, 

 and both minute. First lateral lobe much smaller than the siphonal lobe, 

 and divided into two principal branches, with a broad, rounded sinus 

 between, the dorsal side provided with three nearly equal spurs, one above 

 the other, and the ventral side with two larger spurs, each of which is min- 

 utely bifurcate at its extremity. Second lateral lobe long and narrow, 

 with a club-shaped extremity, marked by three minute points on the dorsal 

 and two on the ventral side. Third lateral lobe minute, narrowly linear, 

 and perfectly simple. Fourth lobe barely perceptible as a slight indenta- 

 tion. First sinus consisting of three nearly equal, cordiform divisions, 

 separated by secondary lobes, the first long, narrow, and simple; the second 

 larger, narrow, and clavate, with four minute terminal teeth. Second sinus 



