Texas Fossils: An Amateur Collector's Handbook

by William Henry Matthews

Plate 31
[TERTIARY] PELECYPODS

PLICATULA × 1 PECTEN × 1 ANOMIA × 1 GLYCYMERIS × 2 CARYOCORBULA × 2 BARBATIA × 1

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

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].