The nautiloids are cephalopods with external chambered shells in which
the _septa_ (dividing partitions) are simple and have smooth edges. This
subclass is represented by a single living genus, _Nautilus_, and a
large number of fossil forms.
In the living _Nautilus_ the shell is composed of calcium carbonate and
is coiled in a flat spiral (fig. 18). The interior of the shell is
divided into a series of _chambers_ by calcareous partitions called
_septa_. The point where each septum joins the inner surface of the
shell is known as the _suture_. These _suture lines_ (fig. 19a) are not
visible from the outside unless the outer shell has been removed, but
they are visible on the internal molds of many fossil cephalopods and
are of great importance in nautiloid and ammonoid classification.
Nautiloids have very simple smoothly curved suture patterns, but
ammonoids are characterized by more complex and wrinkled sutures (fig.
19d).
Although the shell of the only type of living nautiloid is coiled, many
of the early forms had straight cone-shaped shells (_Orthoceras_, Pl.
32), and these are common in some of the Pennsylvanian formations of
Texas. Fossil coiled nautiloids may be collected in certain of the
Cretaceous and Tertiary strata of the State, but their remains are not
common. _Cymatoceras_ (Pl. 32) is a coiled fossil nautiloid from the
Cretaceous of north Texas.
[Illustration: Plate 24
PENNSYLVANIAN PELECYPODS]
SCHIZODUS × 1
MYALINA × ½
ASTARTELLA × 1
NUCULOPSIS × 1
ALLORISMA × 1
NUCULANA × 1
YOLDIA × 1
PINNA × ½
[Illustration: Plate 25
CRETACEOUS PELECYPODS]
PROTOCARDIA × 1
ALECTRYONIA LUGUBRIS × 1
PLICATULA × 1
PECTEN × 1
[Illustration: Plate 26
CRETACEOUS PELECYPODS]
GRYPHAEA WASHITAENSIS × 1
GRYPHAEA GRAYSONANA × 1
INOCERAMUS × 1
TRIGONIA × 1
[Illustration: Plate 27
CRETACEOUS PELECYPODS]
EXOGYRA ARIETINA × 1
EXOGYRA LAEVISCULA × 1
NEITHEA × 1
EXOGYRA PONDEROSA × 1
EXOGYRA TEXANA × 1
[Illustration: Plate 28
CRETACEOUS PELECYPODS]
PACHYMYA × ½
OSTREA CARINATA × 1
OSTREA QUADRIPLICATA × 1
PHOLADOMYA × 1
[Illustration: Plate 29
TERTIARY PELECYPODS]
LIMA × 1
OSTREA LISBONENSIS × 1
PITAR × 1
VENERICARDIA BULLA × 1
PACHECOA × 2
PHOLADOMYA × 2
OSTREA SELLAEFORMIS × 1
CRASSATELLA × 1
[Illustration: Plate 30
TERTIARY PELECYPODS]
ORTHOYOLDIA × 1
TELLINA × 1
VOKESULA × 2
NUCULA × 2
VENERICARDIA × ½
[Illustration: Plate 31
TERTIARY PELECYPODS]
PLICATULA × 1
PECTEN × 1
ANOMIA × 1
GLYCYMERIS × 2
CARYOCORBULA × 2
BARBATIA × 1
[Illustration: Fig. 18. Morphology and principal parts of the pearly
nautilus. (a) Exterior view of a Recent shell. (b) Sectioned view of the
same shell to show internal structures.]
b
Living chamber
Aperture
Septa
Siphuncle
Protoconch
Chamber
[Illustration: Fig. 19. Characteristic features of the various types of
cephalopod sutures. (a) Nautiloid type. (b) Goniatite type. (c)
Ceratite type. (d) Ammonite type.]