Page:Popular Science Monthly Volume 77.djvu/153

Rh The Protozoa afford in the Carboniferous Fusulinidæ and in the Tertiary Nummulinidæ forms with very different morphological characters from those living today, while the numerous extinct species of the Lituolidæ and Textularidæ in the Cretaceous and of the Miliolidæ and Globigerinidæ in the Tertiary have greatly widened our knowledge of the entire subkingdom.

The Cœlenterata in the Paleozoic Tabulata and Graptoloidea show types so different from living forms that the systematist has never been able to satisfactorily assign them to a position within the limits of the phylum. Many external and internal characters appear that are quite unknown in later forms. On the other hand, the paleontological subclass of the Tetracoralla long imperfectly understood is now regarded with a fuller knowledge of the morphology as affording the probable ancestors of the later Hexacoralla.

The Echinodermata have furnished two classes, the Cystoidea and the Blastoidea, unknown after the Paleozoic, whose morphology aids very materially in an interpretation of later and more highly differentiated forms among the Pelmatozoa. Thus the cystoids, which have been regarded as the ancestral type from which the crinoids have sprung, afford forms like the Camarocystites, in which the arms are similar to those of the crinoids although the calyx plates are irregularly arranged and thus cystoidean in character. Both the Asterozoa and Echinozoa are represented in the fossil state by many species that greatly widen our knowledge of the morphology of this group. Take for example, the Echinocystites, regarded as belonging to the Palechinodea which has a valvular pyramid of calcareous anal plates so highly characteristic of the cystoids.

The Molluscoidea, to which phylum belong the Bryozoa and Brachiopoda, would be but imperfectly understood from a morphological standpoint but for the vast number of fossil forms. The Brachiopoda have been estimated to have less than 150 living species, while probably more than 6,000 fossil species have been described. Of the 31 families only 7 have living representatives. We are dependent, therefore, largely on the fossil forms for our knowledge of the morphology of this class.

The Mollusca with their varied forms, although so well represented to-day, have furnished in the fossil state one of the most interesting and important orders in the animal kingdom, the Ammonoidea with its 5,000 and more species ranging from the Devonian to the Cretaceous. Even the allied Nautiloidea, although containing living forms, attained its chief development in the Paleozoic, and it is from these ancient forms that we obtain our chief knowledge of the morphology of this group with their early straight and irregularly coiled types.

The Arthropoda afford in the Paleozoic the important groups of the trilobites and euripterids, forms that have aided greatly in the