44 THE PALEONTOLOGIC RECORD 



The Protozoa afford in the Carboniferous FusulinidaB and in the 

 Tertiary NummulinidaB forms with very different morphological char- 

 acters from those living to-day, while the numerous extinct species of 

 the Lituolidas and Textularidse in the Cretaceous and of the Miliolidae 

 and Globigerinidag in the Tertiary have greatly widened our knowledge 

 of the entire subkingdom. 



The Ccelenterata 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 re- 

 garded 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 differen- 

 tiated 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 char- 

 acteristic of the cystoids. 



The Molluscoidea, to which phylum belong the Bryozoa and Brachio- 

 poda, would be but imperfectly understood from a morphological stand- 

 point 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 inter- 



