312 BRITISH PALAZOZOIC FOSSILS. [CrepHALopopa. 
The family contains the following genera and subgenera: 1, Nautilus; 2, Cyrtoceras; 3, Aturia; 
4, Aganides ; 5, Clymenia; 6, Subclymenia ; 7, Orthoceras ; (8, Cycloceras ; 9, Poterioceras ; 10, Loxoceras.) 
Genus. CYRTOCERAS (Gold.) 
Gen. Char.—Shell conic, gently arched ; siphon at the outer edge or nearly so; septa simple. 
The similarly-shaped species with subcentral siphon form the genus Aploceras (D’Orb.) 
CYRTOCERAS MULTICAMERATUM (f/all). 
Ref.—Pal. New York, t. 42. f. 4. 
Sp. Ch.—Length of specimen nearly two inches, gently arched to within half an inch of the apex, which 
is abruptly curved to less than half a circle; antero-posterior diameter at mouth about six and half lines, 
moderately compressed laterally ; section broad, oval; siphon obscure, but apparently near the external narrow 
end of the oval septa; septa very numerous, five in the space of two lines, at two to three lines in diameter 
at about four lines from the apex. 
This species is remarkable for the great approximation of the slightly convex septa, which agree exactly 
in proportional distance, by measurement, with Hall’s figure. 
The American locality is ‘‘ Lower Shaly beds of the Trenton limestone at Middleville.” 
Position and Locality —In the shaly beds of the concretionary Lower Bala limestone at Knockdollian 
mountain, three miles from Ballintrae, Ayrshire. 
Genus. ORTHOCERAS (Breyn.) restricted. 
Syn. = (Molossus + Achelois Montf.) > (Huronia + Ormoceras Stokes = Actinoceras Bronn.) 
Gen. Char.—Shell conical, straight or nearly so, having the greater part of the posterior end traversed 
hy convex, transverse septa, with simple edges, at right angles to the long axis of the shell; siphon cal- 
careous, central or slightly eccentric, cylindrical or dilated between the chambers; external surface even. 
In my Synopsis of the Carboniferous Fossils of Ireland (p. 6) I have proposed to restrict the term Orthoceras 
to such of the above as had perfectly central simple siphons, I now find however that it is absolutely necessary 
to include also the species which have it more or less eccentric, but agreeing in other respects; I also find 
that the gradation is so insensible from the species with narrow, simple, siphons to those in which it is 
more or less dilated between the chambers (forming the genus Actinoceras of Bronn and Stokes), or those 
in which the dilatations correspond with the septa and the constriction with the intervening spaces (forming 
the genera Huronia and Ormoceras of Stokes), that it is impossible even to separate them as subgenera. 
The genus Melia or Thoracoceras of Fischer is founded (Bulletin de la Soc. des Nat. de Moscou, Vol. 
XVII. p. 755) on Orthoceratites with lateral siphons, but dissimilar generic characters in other respects ; 
his notion that the septa do not entirely surround the siphon not being applicable to the bulk of his species, 
and especially the 7. vestitum, the type of his genus has a small only submarginal siphon, agreeing in other 
respects with Orthoceras, and although the group is adopted by M. d@Orbigny in his “ Prodrome” for some 
species having the latter character, yet they are very dissimilar among themselves, and are not analogous 
to the greater number of Fischer’s species ; it does not seem to me possible consequently to recognise the genus. 
Restricting therefore Orthoceras to plain species with central or slightly eccentric siphons, and the septa at right 
angles to the general axis, we get a second very good subgenus or genus, which in the before-mentioned work 
I have published under the name of Lozxoceras (M°Coy), in which the general form is the same, but the septa 
are oblique to the long axis, and have a deep wave in their edges on each side, and the siphon is either 
marginal or submarginal, varying however as in Orthoceras in thickness or inflation between the septa. 
The name Lndoceras is provisionally applied by Hall to such Orthoceratites as have young septate tubes 
