THE PALEONTOLOGIC RECORD 147 
The Protozoa afford in the Carboniferous Fusulinide and in the 
Tertiary Nummulinide forms with very different morphological char- 
acters from those living to-day, while the numerous extinct species of 
the Lituolide and Textularide in the Cretaceous and of the Miliolide 
and Globigerinide in the Tertiary have greatly widened our knowledge 
of the entire subkingdom. 
The Celenterata 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- 
