316 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



small digitations ; the first two lateral branches above 

 these are small, opposite, very diverging, and bifid or 

 digitate ; and the third pair very small, and apparently 

 simple. The dorsal saddle is as long as the dorsal lobe, 

 but narrower, and has three or four short obtusely 

 rounded branches on each side. The superior lateral 

 lobe is nearly as large as the dorsal saddle, and has three 

 subequal branches at the extremity, — that on the dorsal 

 side being bifurcate, with digitate divisions ; and the 

 middle, and other lateral divisions, are provided with three 

 or more small digitations each. The inferior lateral lobe 

 is much smaller than the superior lateral, and has much 

 the same form, excepting that its terminal division is 

 proportionally larger, and the principal lateral division on 

 the dorsal side is not so deeply divided. The ventral lobe 

 is a little smaller, but in other respects very similar to the 

 inferior lateral lobe ; between it and the umbilicus there 

 appears to be one or two smaller auxiliary ventral lobes, 

 which seem to show a tendency to branch in the same way 

 as the principal ventral lobe. 



The specimen from which the foregoing description was 

 made out, is evidently a young shell ; consequently, adult 

 individuals of the same species may be expected to possess 

 much more distinct costas. The lobes and saddles of the 

 septa, in old shells, will also be found much more deeply 

 divided and more complex, but the mode of branching 

 probably remains the same from the time the principal 

 divisions are formed. 



As the specimen described was found in the matrix 

 filling the umbilicus of A. Barnstoni (being only 0*67 inch 

 in its greatest diameter), it might be supposed by those who 

 know how widely the Ammonites sometimes vary at dif- 

 ferent ages, that it may be the young of that species. It 

 presents fundamental differences, however, in the mode of 



