PA ASU Ne TEE 
PLACENTICERAS. 
Fies. 1, 2. Placenticeras pseudoplacenta var. occidentale Je AINE area a aaa Ne ae MEG eae nes 217 
Fig. 1. A specimen from the Upper Missouri River; Coll. Columbia University, 
No. 10622G. 
2. Suture of a young specimen from Horton’s mill, Dallas County, Tex.; 
Coll. Mus. Comp. Zool., Cambridge. 
IEE, GalG, JAaemaiearas cai hOtels ISVEIN, soo eeacocecd es anoneses aoc cosaenes seunnoseneaees Here 221 
Fig 3. Side view of small fragmentary specimen from Clifford, Nebr., in Coll. 
Mus. Comp. Zool., Cambridge, enlarged. 
Section of same showing venter of earliest whorls, >< 3. 
Ces 
Center of same, about 63 diameters. 
First two sutures and beginning of siphuncle of same specimen, much 
for) 
enlarged. 
Beginning of siphuncle of same after etching with acid, still further 
=I 
enlarged. 
8-10. Early stages obtained by breaking down a larger specimen from the 
Black Hills, South Dakota, in my collection; enlarged about 5 diameters. 
11-14. Complete sutures from dorsum to venter of same stages as figs. 8, 9, 10. 
Fig. 11 is earliest suture visible on fig 9; fig. 12 is on last quarter of 
same whorl; fig. 13 is last suture on fig. 10, and fig. 14 is nearly a volution 
earlier. 
15. Dorsal (internal) sutures at diameter of 15 mm. 
Same at diameter of 40 mm. The last two figures may not be from same 
specimen as figs. 8-14. 
Montana group, Upper Cretaceous. 
342 
