Logan.] The Invertebrates, Benton Group. 463 



Placentireras placenta Dekay. 



Ammonites placenta Dekay, 1828, Ann. N. Y. Lye. Nat. Hist., vol. n, p. 278, 



pi. v, f. 2, and of various authors. 

 Ammonites {Placenticeras) placenta Meek, 1S70, U. S. Geol. Surv. Terr., 



p. 297, and 1S70, Proc. Amer. Phil. Soc, vol. xi, p. 429. 

 Plaeenticeras vla°enta Meek, 1876, U. S. Geol. Surv. Terr., vol. ix, p. 465; 



Stanton, 1892, Bull. U. S. Geol. Surv., p. 169, pl.xxxix, ff. 1-3. 



Meek's description : " S'hell lenticular in form, attaining a 

 large size ; umbilicus small ; volutions deeply embracing, com- 

 pressed laterally, with sides converging from near the umbilicus 

 to near the periphery, which is very narrowly truncated and 

 flattened or a little concave, with its smooth margins becoming 

 more obtuse with age ; aperture narrowly sagittate ; surface 

 generally smooth or only showing very obscure traces of curved, 

 transversely elongated prominences on each side, with some- 

 times a row of very small indistinct nodes around the umbilicus ; 

 in young exfoliated shells also usually showing small, faintly 

 denned, divaricating corrugations, directed backward around 

 the outer half of each side. Large examples attain two feet or 

 more in their greatest diameter. Young specimens, three and 

 seventy-hundredths inches in breadth, show a thickness of 

 about ninty -hundredths of an inch, while large individuals are 

 proportionally thicker, and on the periphery become more ob- 

 tuse. Septa with twelve lateral lobes, and as many sinuses on 

 each side, in large examples crowded and very complex." 



I obtained a large fragment of this species from the Blue Hills 

 shales in the vicinity of Williams' Butte, in Mitchell county. 

 It is the only specimen known to have been collected from the 

 Fort Benton of this state. 



Prion or yd as Wyoming ensis Meek. Plate cvi. 



Ammonites serratoearinatus Meek, 1871, Proc. Am. Phil. Soc, vol. xi, 



p. 429. 

 Ammonites (Pleuroceras?) serratoearinatus Meek, 1871, Ann. Rep. U. S. 



Geol. Surv. Terr, for 1870, p. 298. 

 Prionocycli's wyomingensis Meek, 1876, U. S. Geol. Surv. Terr., vol. ix, 



p. 452; White, 1880, Ann. Rep. U. S. Geol. Surv. Terr, for 1878, p. 35. pi. 



xv, ff. la-e: Whitfield, 1880, Geol. Black Hills Dak., p. 440, pi. xiv, ff. 1-3; 



Stanton, 1893, Bull. U. S. Geol. Surv. No. 106, p. 171, pi. xl, ff. 1-4. 



Original description : " Shell attaining a rather large size; 

 discoid, with periphery provided with a very narrow, prominent, 



23- iv 



