WMITEAVE6.] CRETACEOUS FOSSILS, NORTH WEST TERRITORY. 183 



Placentioeras placenta, Dekay. (Sp.) 



Ammonites placenta, Dekay. 1828. Ann. N. York Lye. Nat. Hist., vol. II, p. 278, 

 pi. 5, fig. 2 (3 by mistake).— Morton. 1829. Journ. Ac. Nat. 

 Sc. Phil., vol. VI, p. 195 ; and Am. Journ. Sc. and Arts, 

 vol. XVIII, pi. 2, figs. 1, 2 and 3 ; also 1834, Synops. Org. 

 Rem. Cret. Form. U. S., p. 36, pi. 2, figs. 1 and 2. 



Placentkeras placenta, Meek. 1876. Eep. U. S. Geo! Surv. Terr., vol. IX, p. 465, pi. 

 24, figs. 2, a, b. 



Battle Elver, Township 40, Eange 13, west of the 4th Principal 

 Meridian, 1885 : a small fragment. 



Sounding- Creek, Township 30, Eange 8, west of the same meridian, 

 1886 ; a single but nearly perfect specimen which measures nearly 

 nine inches in its greatest diameter. 



CBUSTACEA. 

 Pal,«astacus (?) ornatus, Whiteaves. 



Plate25, fig. 3. 



Palieaatacus (f) ornatus, Whiteaves. 1887. Geol. and Nat. Hist. Surv. Can., Ann. 

 Rep., N. Ser., vol. II., p. 161 E. 



The foregoing was suggested as a provisional name for a rather re- 

 markable specimen of a long tailed decapod, which evidently belongs 

 to the family Astacomorpha of Zittel. Of the Cretaceous representatives 

 of this family, it seems to come nearest to such genera as Palceastacus 

 and Hophparia, though it differs from each in some important parti- 

 culars. In many respects it appears to the writer to be still more 

 nearly related to the recent and fresh-water genera Astacus and 

 Camharus, but there is good reason for supposing that it will eventually 

 prove to be the representative of a new generic type, which at present 

 there is not sufficient material to define satisfactorily. 



JSTearly the whole of the under surface of the cephalothorax of the 

 specimen is bui-ied in the matrix, the front margin of the carapace is 

 very imperfect, the tail fin as well as the under part of the five abdo- 

 minal segments are broken off, and only small portions of the pinching 

 claws and of the other ambulatory legs are preserved or exposed. 



The carapace is moderately convex or slightly depressed, and not 

 quite twice as long as broad. It is divided into two nearly equal parts 

 by a single, well marked and deeply impressed neck furrow, which is 

 arched forward in a shallowly concave curve. Behind this furrow 

 the lateral margins of the carapace are slightly expanded, the test in 

 the branchial region is moderately inflated, and the posterior margm 

 3 



