470 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



anterior ends, and the lobes more or less distinctly tripartite at their poste- 

 rior ends, excepting the last or twelfth one, which is very small and nearly 

 or quite simple. 



This shell seems to lie almost exactly intermediate, in most of its char- 

 acters, between Ammonites placenta of Dekay, and A. syrtalis of Morton ; that 

 is, it has the same form as A. placenta, with which it also agrees in showing 

 no tendency to form costa\ or obscure ridges, on the sides; while it differs 

 from that species, and agrees with A. syrtalis, in having a row of small com- 

 pressed nodes along each margin of the periphery, and two rows of tubercles, 

 or prominences, around each side, though the lateral tubercles are more 

 rounded than they are represented on Dr. Morton's figure of A. syrtalis. 



In first, publishing a notice of this shell, in the Proceedings of the Acad- 

 emy of Natural Sciences, in 1860, we referred it to A. placenta, but at the 

 same time called attention to its intermediate characters between the typical 

 form of that species and A. syrtalis, Morton, and proposed for it the name 

 intercalaris, as a variety of A. placenta. 



On subsequently working out the details of one of its septa, as repre- 

 sented by the figure on plate 23, it was found to present some rather 

 important differences from the septa of A. placenta, as may be seen by com- 

 paring figure 1, c, of plate 23, with the cut of a septum of the latter given in 

 connection with the description of the same on a preceding page; tljat is, 

 the first lateral lobe of the form here under consideration is tripartite, and 

 narrow at its posterior end, instead of being bipartite, with the two terminal 

 branches spreading. Some other less marked differences might also be 

 pointed out, but these, at least in part, might be accounted for by the differ- 

 ences in the sizes of the specimens from which the drawings of the septa 

 were made out ; that of the form here under consideration being less than 

 half as large as the specimen from which the cut of the septum of A. placenta, 

 given on a preceding page, was made. 



At that time I had no means of comparing the septa of this shell with 

 those of A. syrtalis ; Dr. Morton's figure of the sutures of his species not being 

 sufficiently accurate for such purposes. On writing to Philadelphia, how- 

 ever, my friend Mr. Gabb was kind enough to work out a very careful draw- 

 ing of one of the septa from Dr. Morton's type-specimens. This I have had 

 photographed on a block of wood, and engraved twice the natural diameter, 

 and inserted here. 



