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.] Subclass Ammonoidea.— The ammonoids are a group of extinct cephalopods which are related to the nautiloids but are characterized by more complex suture patterns. Members of this subclass have an external partitioned shell which is straight, curved, or spirally coiled (Pl. 33). This group of cephalopods first appeared in Devonian time, became extremely abundant and varied during the Mesozoic, and was extinct by the end of the Cretaceous period. Most Paleozoic ammonoids are characterized by a combination curved and angular suture pattern, and this type of suture pattern is referred to as _goniatitic_ (fig. 19b). Sutures that are curved and crenulated (marked in places by a series of tooth-like indentations) are known as _ceratitic_ (fig. 19c). Ceratitic sutures first appeared in the Mississippian, became increasingly abundant during the Triassic but were much less abundant during the Cretaceous. The _ammonitic_ suture has a very complexly subdivided pattern (fig. 19d). Cephalopods with ammonitic sutures range from Pennsylvanian to Cretaceous in age and were the most abundant cephalopods of the Mesozoic. [Illustration: Plate 32] [Illustration: PENNSYLVANIAN CEPHALOPODS] PHANEROCERAS × 1 ORTHOCERAS × 1 [Illustration: CRETACEOUS CEPHALOPODS] METOICOCERAS × ½ CYMATOCERAS × ½ [Illustration: Plate 33 CRETACEOUS CEPHALOPODS]