52 NAUTILID^. 



into its shell, as in the Nautilus. That the shell was external is 

 indicated by the colored bands preserved on 0. anguliferus. 

 These shells sometimes grew to a great size ; a specimen in the 

 collection of Mr. Tate of Alnwick, England, must have been six 

 feet long when perfect. Newberry, in the Palaeontology of Ohio, 

 estimates another species, 0. Titan, to have weighed " some 

 tons." The aperture is sometimes so contracted that species 

 two feet in length have a diameter of only one inch at the mouth. 



CAMEROCERAS, Conrad, 1842. (Melia, Fischer; Sannionites, 

 Fischer.) Siphuncle lateral, sometimes very large (simple?). 

 Casts of the large siphuncles were called Hyolites by Eichwald. 

 Distr. — 27 sp. L. Silurian to Triassic (?) ; N. America, Europe. 



ACTINOCERAS (Brown), Stokes. Siphuncle very large, inflated 

 between the chambers and connected with a slender central tube 

 by radiating plates. 6 sp. L. Silur. to Carb. ; N. America, Europe. 

 0. Bichardaoni, Stokes (xxix, 78). 



ORMOCERAS, Stokcs, 1838. Siphuncular beads constricted in 

 the middle, so that the septa appear as if united to the centre of 

 each. Probably identical with Actinoceras. Distr. — 3 sp. L. 

 Silurian to Devonian ; N. America. 0. Bayjieldi, Stokes (xxix, 

 84). 



HURONiA, Stokes, 1823. (Discosurus, Hall, 1852.) Shell ex- 

 tremely thin, membranous or horny (?). Siphuncle very large, 

 central, upper portion of each joint inflated, connected with a 

 small central tube of radiating plates. Usually the siphuncle 

 only is preserved. Dr. Bigsbj^ observed specimens six feet in 

 length. Doubtfully distinct from Actinoceras. Distr. — 3 sp. 

 L. Silui'ian ; Drummond Isl., Lake Huron. H. vertebralis, Stokes 

 (xxix, 85). 



AULACOCERA'^, Hauer. Shell much thickened, longitudinally 

 furrowed, with two deep lateral sulcations ; siphon very small, 

 marginal. Distr. — 4 sp. Upper Triassic ; Austria. 



BATHMOCERAS, Barraudc, 1865. Part of the body-chamber 

 occupied by imbricating plates, decreasing in horizontal exten- 

 sion from below upwards ; siphuncle a series of superimposed 

 funnel-shaped tubes. Distr. — 4 sp. Silurian; Bohemia, Sweden, 

 Lake Huron. 



ENDOCERAS, Hall, 184*7. (Conotubularia, Troost ; Diploceras, 

 Conr.) Shell extremelj^ elongated, cylindrical. Siphuncle very 

 large, cylindrical, lateral ; thickened internally by repeated 

 layers of shell, or partitioned off by funnel-shaped diaphragms. 

 Distr. — 12 sp. L.Silurian; New York, Europe. 



TRETOCERAS, Salter, 1858. (Diplocei'as, Salter, not Conrad, 1856 ; 

 Nothoceras, Eichw., 1859.) Founded on O.bisiphonatiim, Sowh. 

 (xxx, 93), from the Caradoc sandstone ( Silurian), Brit,, in which 

 the septa are apparently perforated b}^ two siphuncles ; one of 

 which is a deep lateral cavity continuous with the terminal 



