1883.] 275 [Hyatt. 



group, and the evolution of the nummuloidal type from the va- 

 rious groups of tubular siphoned and straight cones. 



Orthoceras, Breynius, should we think be confined to straight 

 and comparatively smooth longicones with simple septa and 

 sutures, it equals group 17 of M. Barrande. The author has 

 met with but two species in North America, though doubtless 

 others may exist, since the extreme smoothness of the shell is 

 easily destroyed. The genus is present in all the paleozoic for- 

 mations and in the Trias. 



Geisonoceras, 1 nobis, includes various groups of the banded 

 longicones of M. Barrande. They fade into true Orthoceras, and 

 yet can certainly be distinguished by the transverse markings or 

 bands, which are formed on the surface of the shell. We include 

 in this series, groups 10, 11, 12, 13, 14 of M. Barrande. The 

 young are either smooth or transversely striated. Type, Gei. 

 (Orth.) rivale, sp. Barr., pi. 209, 216, 387. The banded longi- 

 cones are directly connected by transitional forms with Cyclo- 

 ceras, and with the banded brevicones of the genus Rizosceras. 

 The characteristic bands of the shells and the position of the 

 siphon in some species make a close approximation to Bactrites. 

 Silurian, Devonian, Carboniferous ? 



Cycloceras, McCoy, Synop. Carb. Foss. Ireland, 1844, includes 

 the transversely striated, Paleozoic longicones, which at some 

 stage of growth have annular costae. The young are invariably 

 smooth, that is, marked only by transverse striae of growth, as 

 in Cyc. (Orth.) Agassizi, sp.fBarr., pi. 281, and the annulations are 

 subsequently introduced. It includes group 9 of M. Barrande. 

 Silurian, Devonian, Carboniferous. 



Kionoceras, 2 nobis, includes the longicones in which the 

 longitudinal ridges are more prominent than the transverse striae 

 or ridges when these are present and are smooth throughout their 

 entire length. Equal to group 4 of M. Barrande ; type, Kion. 

 (Orth.) doricum, sp. Barr., pi. 269. Silurian, Devonian, Carbon- 

 iferous. 



Thoracoceras, Eichw. Bull. Soc. Imp. de Nat. de Mosc, 1844, 

 p. 761, was a name substituted for Melia proposed in 1829 by the 



1 TeCo-ov, a cornice. 



2 Ktwv, a column. 



