XIV 
INTRODUCTION. 
able for the great width of the umbilicus and the largo size of the 
central perforation, as compared with the diameter of the shell 
Many of the Triassic species of Temnoclieilus and Pleuronautilus 
still retain a large umbilicus, exposing the whole of the volutions ; 
but the central perforation in them has become much reduced in 
size, though conspicuous in Pleuronautilus Mosis, Mojs. Other 
Triassic forms, such as Nautilus hremhanus^ Mojs. N. noricus, 
Mojs. N galeatus, Mojs. N. triadicus, Mojs. ®, N. (Hercoglossa) 
Sauperi, Fr. v. Hauer &c., have the umbilicus closed, or nearly 
so, as in many of the Jurassic, Cretaceous, and Tertiary forms, and 
one of the living species (iV. pompilius). In all of these the whorls 
are deeply embracing. 
Some species resemble others chronologically remote from them ; 
thus — Nautilus Jieocagonus, J. de C. Sowerby ® { = N. giganteus, 
d’Orb.), of the Jurassic, resembles in its sulcated periphery tlic 
Carboniferous genus Discites, and similarly Nautilus triangularis, 
Montfort®, in respect of its acute peripherj^ recalls Nautilus IPhaco- 
ceras'] occystomus, Phillips Amoug such morphological equivalents 
Professor Hyatt notes “ the extraordinary likeness of Chjdo- 
See Waagen, Mem. Geol. Surv. India — Palgeont. Indica — Ser. xiii. Salt- 
Range Fossils, 1879, vol. i. pt. i. p. 58, pi. v. fF. 2a, 2b, Nautilus ophioncus, 
Waagen, in which the central space is 1^ inches in diameter, the greatest 
diameter of the shell being only about 6 inches ! Another species with a very 
wide central space is Nautilus Meyerianus, de Koninck (Calc. Carb. pt. i. 1878, 
pi. xxix. ff. 1, 2, 3). In this the central perforation measures 1 inch 1 line ; 
while the greatest diameter of the shell is 4 inches 2 lines. 
^ See infra, p. 135, fig. 22. 
^ Die Ceph. der Mediterr. Triasprov. 1882, p. 283, Taf. xc. figs. 4n, 46. 
Das Gebirge um Hallstatt (Abhandl. der k.-k. geol. Reichsanst. Band vi.), 
1873, p. 25, Taf. xi. figs. 1, a-c, 2. 
^ Ibid. p. 26, Taf. xii. figs. 1 a, b, Taf. xiii. figs. 1 «, 1 6, 3. 
Ibid. p. 27, Taf. xiv. figs. 1, 2 a-d, 3 a, b, 4. 
Haidinger’s Naturwiss. Abhandl. 1846, Band i. p. 26, Taf. i. figs. 1-4. 
® Min. Conch. 1826, vol. vi. p. 55, pi. dxxix. fig. 2. 
^ Pal. Frang., Terr. Cret. 1840, vol. i. p. 79, pi. xii. 
Geol. Yorkshire, 1836, pt. ii. p. 233. 
n (t Morphological equivalence ” is a phrase used by Prof. Hyatt , and he thus 
explains it;— “In the different genetic series of a type derived from one 
ancestral stock there is a perpetual recurrence of similar forms in similar 
succession, which are usually called representative and often falsely classified 
together,- though they really belong to divergent genetic series.” — “ Genesis of 
the Arietidse” (Smithsonian Contributions to Knowledge), 1889, Preface, 
p. viii. 
