Hyatt.] 290 April 4, 



tilian shells similar to the above, but of larger size with a narrow 

 impressed zone on the dorsum, and a corresponding undivided, 

 narrow, dorsal lobe. The sutures have similar ventral, and lateral 

 lobes, but there are a pair of slight lateral saddles near the 

 shoulders. The whorl in section lias a narrower abdomen than in 

 Apsidoceras, and longer abdomino-dorsal diameter, and is more 

 compressed or shield-shaped, rather than depressed or triangular 

 in section. Type, Titan. (Naut.) ponderosum, sp. White, IT. 

 S. Geol. Surv. Final. Rep. on Nebr. Hayden, p. 236, pi. 3. Nat. 

 Mus. 



Ephippioceras, 2 nobis, includes Carboniferous forms with sub- 

 acute prominent ventral saddles, broad lateral lobes, sub-acute 

 lateral saddles near the shoulders, and broad, shallow dorsal 

 lobes. In the American species, and perhaps in all, there is a 

 slight dorsal saddle in the centre of this lobe. The septa in all 

 species are creased, or raised into a median ridge between the 

 two saddles. The aperture of Ephip. clitellarium reminds us of 

 Pteronautilus in its shallow, acute ventral sinus. Type is Ephip. 

 ferratum, sp. Owen, Geol. Kent., Vol. 3, p. 574, pi. 10, fig. 2, a 

 species closely allied to Ephip. (Naut.) bilobatum Sow., De Kon. 

 Calc. Carb., pi. 9 but is less involute. The siphon is below the 

 centre in the late adolescent stages, and above the centre, or 

 central in adults of the type species. 



Pteronautilus, Meek, Pal. Up. Missouri, Smith. Contr., Vol. 

 14, p. 64, includes but one Dyassic species. This has completely 

 involute whorls, and an aperture extended laterally into wings. 

 The ventral sinus of the aperture is singularly acute resembling 

 in this respect that of Ephip. clittellarium. This slight indication 

 of affinity enables us to place the genus provisionally in this 

 series. Type, Pter. (Naut.) Sebachianus, sp. Gein. Dyas, p. 43, 

 pi. 11. 



Trigonoceratidae. 

 The adults of the radical species, and early stages of descend- 

 ent forms have whorls similar to those of the Apsidoceratidae. 

 There are, however, longitudinal ridges along the edges of the 

 sulcated abdomens in the adults of the radicals, and in the young 

 of descendent forms these are repeated and then followed in 



Titov, Titan. 'E^Cttttiov, a saddle. 



