170 
NATITILOIDEA. 
fossils upon the Black Eiver.” This ohservation, though applicable 
to some of Hr. Stokes’s Orthoceratites, is erroneous as regards 
Ormoceras Backii, which is from the Niagara Formation (Silurian) 
of Drummond Island, Lake Huron. 
Actinoceras [“ Conotubularia^’~\ Cuvieri, Troost\ is easily dis- 
tinguished from A. Bicjshyi by its much closer septa and its 
relatively smaller siphuncle. 
Though Hall Qoc. cit. p. 56) admits that ho had little doubt of 
the identity of his species with Actinoceras, Bronn, “ as figured by 
Stokes,” he includes two species which are certainly distinct, 
namely, A. Baclcii and A. Cuvieri. 
Billings imj)lied his belief in the identity of A. Bigsbyi with 
“ Ormoceras ” tenuifilum by making the latter a synonym of the 
former ^ ; and with this view I entirely agree. 
Eichwald refers doubtfully to this species an Actinoceras found in 
the Or^/ioc^ras-limestone of Hesenberg and Nyby in Esthonia. 
He thinks that Ortlwceras Jceve, Schmidt ^ may also be identical 
with A. Biyshyi. But in A. Iceve the greater diameter of the 
siphuncular segments is said to be only one fourth that of the 
chambers, whereas in A. Biyshyi this diameter considerably exceeds 
one half. The segments are also oblique to the long axis of the 
siphuncle, which is not the case in A. Biyshyi. 
One of the specimens in the National Collection is interesting 
from its exhibiting a peculiar mammillated structure upon the 
surface of the cast, which is considered by Barrande to be the marks 
made by the mantle of the mollusk. Illustrations of these appear- 
ances are to be found in the ‘ Syst. Sil. de la Boheme,’ vol. ii. 
pis. ccxxviii., ccxxix. The specimen showing them is from the 
Trenton Group of Eagle ClifiP, Iowa. 
Horizon. Black Eiver ( = Llandeilo Limestones ?), Trenton ( = Bala 
Limestone Group), Galena Limestone (Upper part of Trenton), Cin- 
cinnati Group (= Lower and Middle Bala?). 
Localities. Igloolik Island, Fox Channel, Arctic America ; Thes- 
salon Island, Lake Huron ; New York State ; (Black Eiver) : Eagle 
Clifi", Iowa; (Trenton): Lake Minnipeg; Eed Eiver, Minnesota; 
(Galena Limestone) : T ersailles, Kentucky ; (Cincinnati Group) 
1 Mem. Soc. Geol. de France, 1838, tom. iii. partie, p. 88, pi. ix. f. 1. 
^ Geol. of Canada, 1863, Appendix, p. 949. 
® Arcbiv fiir die K'aturknnde Livlands, p. 194 (Dorpat, 1858). 
^ There is some doubt as to the identity with the present species of the 
specimens from the last three localities, as they consist only of fragments of 
the siphuncle. These are remarkable for their size, the largest measuring 
2^ inches in its greatest diameter. 
