TEXANITES × ½ ACANTHOCERAS × ½ TURRILITES × ½ DUFRENOYIA × ½ OXYTROPIDOCERAS × ½ BACULITES × ½ BELEMNITE × ½ Ammonoids are locally abundant in many of the fossiliferous rocks of Texas and are among the more useful Mesozoic guide fossils. Goniatites may be found in the Pennsylvanian of north-central and Trans-Pecos Texas, and ammonoids with the ceratitic suture pattern can be collected from the Lower Cretaceous of many parts of the State. Cephalopods exhibiting the typical ammonitic suture pattern are abundant in many of the Cretaceous rocks of Texas, and these fossils have contributed much toward an understanding of the Cretaceous strata of this State. Subclass Coleoidea.— These are squid-like cephalopods characterized by an internal shell or no shell at all. Included in this group are the squids, cuttlefish, octopuses, and the extinct belemnoids, but of these only the belemnoids are useful fossils. Members of this subclass range from Mississippian to Recent in age. Order Belemnoidea.— The belemnoids appear to be the oldest and most primitive of the coleoid cephalopods. Their earliest known occurrence is in rocks of Mississippian age, and they were particularly abundant during the Mesozoic. They became extinct at the end of Cretaceous time but have left considerable evidence of their existence in the Mesozoic strata of many parts of the world. Certain forms, because of their abundance and relatively short geologic range, are excellent guide fossils. Belemnoids have been found in the Upper Cretaceous of Texas (Pl. 33) but in general are rare or unknown in most Texas formations. Phylum Annelida Members of the phylum Annelida include the segmented worms such as the common earthworm. Annelids are marine, fresh water, or terrestrial and have apparently been common through much of geologic time. Because of their lack of hard parts, most of these worms have left little direct fossil evidence of their activities in the geologic past. Some annelids secrete straight or coiled calcareous tubes, and fossil worm tubes of this sort (fig. 20) are commonly found attached to brachiopods, mollusks, and other objects. Tubes of this nature have been reported from Paleozoic, Mesozoic, and Cenozoic rocks in Texas. [Illustration: Fig. 20. Types of typical annelid worms. (a) _Serpula_ (×1) (b) _Hamulus_ (×2). (c) _Spirobis_ (×15).] Some annelids have small chitinous jaws and teeth which also may be preserved as fossils. These dental structures are called scolecodonts and are microfossils. Phylum Arthropoda