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. 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]