312 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



implantation ; after which they become of uniform size, 

 and arch gently forward in passing over the dorsum. 



The septa are deeply divided into five principal lobes 

 and six saddles, which are crowded together, and va- 

 riously branched and subdivided. The dorsal lobe is a 



Ammonite Barnstoni. Side view, showing the deep umbilicus. 



little longer than wide, and has three branches on each 

 side, the two terminals of which are nearly straight and 

 parallel ; the first two lateral branches above these, are 

 nearly of the same size, but more diverging ; while the 

 third pair are much smaller — and all sharply digitate, 

 and more or less subdivided. The dorsal saddle is longer 

 than wide, contracted in the middle, and irregularly di- 

 vided into four unequal branches, the two terminals of 

 which are subdivided into two branchlets each, and all 

 obtusely digitate, and variously sinuous in the margins. 



