IXTRODTJCTIOX. 
xni 
Sub^^euus Aipoceras, Hyatt, 1S83. {Infra, p. 08.) 
Subgenus Trigoxoceras, M‘Coy, 1844. p. 70.) 
JIercoceras, Barrande, 1805. {Infra, p. 74.) 
B.arraxdeoceras, Hyatt, 1883. {Infra, p. 77.) 
Discites, H^Coy, 1844. {Infra, p. 86.) 
Subgenus Phacoceras, Hyatt, 1883. {Infra, p. 98.) 
Ephippioceras, Hyatt, 1883. {Infra, p. 100.) 
Cceloxautilus, Foord, Xov. 1889. {Infra, p. 105.) 
Pleuroxautilus, Mojsisovics, 1882. {Infra, p. 134.) 
Temxocheilus, 5PCoy, 1844, emend. Meek, 1876. {Infra, p. 142.) 
Subgenus Cextroceras, Hyatt, 1883. {Infra, p. 163.) 
SoLExocHEiLUS, Meek and Worthen, 1870, emend. Hyatt, 1883. 
{Infra, p. 165.) 
Nautilus, Breyn, 1732. {Infra, p. 179.) 
Subgenus BLercoglossa, Com-ad, 1866, emend. Meek, 1876. 
{Infra, p. 180.) 
Subgenus Clydoxautilus, Mojsisovics, 1882. {Infr^, p. 182.) 
Aturia, Bronu, 1838. {Infra, p. 335.) 
The family Nautilidce, as represented in the British Museum 
Collection (which is remarkably rich in representative forms), runs 
numerically very close to that of the Orthoceratidce ; both of them 
having, roughly speaking, about 180 species assigned to them. A 
series of Nautilus-like forms ranges from the Silurian {Barrandeo- 
ceras) through all the remaining geological periods, and leaves a 
representative {Nautilus) in the present seas. Modifications of the 
Nautilus type are, as might be expected in such a vast period of 
time, not unfrequent, and they have given rise to the creation of 
several well-marked groups or genera. The latter are, as a rule, 
based upon the external form of the shell. Thus, in Barrandeoceras 
the whorls are few, flattened, and very slightly embracing; in 
Gyroceras they are numerous, elliptical or subtriangular in section, 
and very loosely coiled ; in Discites they are numerous, compressed, 
and more or less sulcated on the periphery. In some genera, such 
as Eyfhipjyioceras in the Carboniferous, and Aturia in the Miocene, 
the sutures afford the chief differential characters. 
Another feature strikingly manifest in the Palscozoic Nautilida3 
is their numerous and very slightly embracing whorls, and con- 
sequently wide umbilicus, with a large central perforation. These 
characters are met with in the earliest rejiresentatives of the groups 
under discussion, viz. the Silurian species Barrandeoceras BoJiemicus, 
B. Sternbergi, &c., and they reach their maximum development in 
the Carboniferous genus Discites, some species of which are remark- 
