30 



for the size of the shell ; in the only specimen of this variety, whose 

 greatest diameter does not much excee<l two inches, the riljs ai-e as wide 

 apart as they are in the largest examjjle of the tyjiical form, which is at 

 least four times its size. 



Septation unknown. 



The exact dimensions of the fossils are as follows: Greatest diameter 

 two inches and nearly four lines; width of umbilicus, about eight lines j 

 height of aperture, five lines ; breadth of do., one inch and three lines. 



Ammonites Lhganianus (?) Form B. 

 Plate VIII., figs. 1 and \it. 



Shell inflated, globose ; the early volutions entirely concealed; umbili- 

 cus very small. 



In the only specimen collected, the outer half of the last whorl is much 

 distorted, and compressed in such a wa}- that the sides are partly forced 

 over the umbilical opening. For this reason the exact amount of the 

 involution, the proportionate width of the umbilicus, and the shape of 

 the aperture cannot be very accurately ascertained. Curiously enough, 

 the distortion does not seem to have much affected the rest of the 

 shell. 



Only one whorl is visible externally ; this is ventricose alike on the 

 siphonal edge and at the sides ; its inward curve is also convex, but rather 

 abruptly so near the sutures. Where the distortion is least, the maximum 

 width of the whorl (or thickness of the shell) is nearl}' equal to three- 

 fourths of the greatest diameter. The aperture is obviously much wider 

 than high, though it is diflicult to estimate in what proportion. The um- 

 bilicus is ver}- small and deep. 



The surface is ornamented with jirimar}', bifurcating costte, and 

 intervening, secondary, simple ribs. About thirty of the former can 

 be counted on the last volution. They commence at the sutures, 

 bifurcate at about a third of the distance across the sides, and re- 

 unite at exactly similar and opposite points. There are no swellings 

 or tubercles on the ribs where they begin to divide. At the commence- 

 ment of the last whoi 1 the ribs bifurcate at a comparatively short distance 

 from the sutures, but near the aperture this distance is much increased. 

 Tlie secondary costa^ usuall}^ alternate with the primaries, but the former 

 are often absent; wdien present the}' encircle the peripherj- but do not 

 extend to the sutures. Two bifurcating ribs are occasionally placed to- 

 gether without any intervening (Uie; or two simple costte nuiy occur be- 



