Additional new Ordovician graptolites and a chitinozoan from Oklahoma

This paper presents the results of a study of specimens of the Ordovician trilobites Flexicalymene senaria (Conrad) and F. meeki (Foerste). The rostrum is described on the basis of isolated silicified specimens. It is unusual in not being a flat plate, but sharply flexed along a transverse line. Serial sections of an enrolled calcareous specimen preserving the hypostome confirm that the wing process on the anterior wing of the hypostome rests in a concavity on the anterior slope of the anterior pit of the cranidium. Abundant perforations, interpreted as the loci of sensory hairs and occurring in many sizes, constitute an important feature of the microstructure of the exoskeletons of these species. They are concentrated on those areas which are most apt to come into contact with other objects in the environment. The larger ones are associated with tubercles of the surface ornament, but smaller ones are distributed without regard to such elevations. The type of preservation strongly influences the abundance, size, and distribution of the perforations appearing in any particular specimen. This limits their potential value for taxonomic purposes.