The exterior of most shells is marked by a [series] of [concentric] growth lines ([fig. 17]a) which mark points of periodic addition of shell material. The external surface of many shells is also marked by various types of ornamentation, such as ribs, nodes, spines, and grooves.
[Fossil] collectors commonly find only one [valve] of the [pelecypod] shell. This is because the shell normally opens when the animal dies, and the valves may easily become separated. Fossil pelecypods are also commonly preserved as external and internal molds, and these are found in [fossiliferous] strata of almost all ages. Some pelecypods of [Pennsylvanian], [Mesozoic], and [Cenozoic] age are found with original shell material that appears to have undergone very little change. Fossil pelecypods are abundant and varied in Texas and are found in most of the fossiliferous formations of the Pennsylvanian, [Cretaceous], and [Tertiary] systems (Pls. [24]-31).
CLASS CEPHALOPODA.—
These are marine mollusks with or without chambered or solid shells which may be internal or external. The living animal possesses a well-developed head with eyes, horny jaws, and many tentacles fused with the foot. Cephalopods are the most advanced of all mollusks and include the squid, octopus, pearly nautilus, and the extinct ammonoids. Members of this class range from [Cambrian] to Recent in age but were much more abundant in ancient seas than they are today. Their remains constitute a very useful group of fossils, particularly in [Paleozoic] and [Mesozoic] rocks.
Most paleontologists have divided the Cephalopoda into three subclasses, the Nautiloidea, Ammonoidea, and the Coleoidea (known also as subclass Dibranchiata and subclass Decapoda); each of these is discussed below.
Subclass Nautiloidea.—
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. 19]a) 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. 19]d).
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.
Plate 24
[PENNSYLVANIAN] PELECYPODS