498 



HISTORICAL GEOLOGY. 



Some of the Graptolites — Hydrozoans — are represented in Figs. 604-009. 

 The texture of the fossil Graptolite was usually thinner than the most delicate 



599-603. 

 600 



603 



Sponges. — Fig. 599, Protospongia tetranema (1); 600, P. mononema ; 601, P. cyatMformis ; 602, P. coronata; 



603, P. Quebecensis. All from Dawson. 



membrane. Only the finest of sediments were therefore adapted to their 

 preservation. The forms with one row of cells, or one-edged (Monoprio- 

 nidae), are represented by the Loganograptus (Figs. 604-606) and species of 



604-609. 



^^M^*^*MMl*MM 



Graptolites. — Fig. 604, Loganograirtus Logani, branches broken oif ; 605, portion of a stem; 606, same, more 

 enlarged ; 607, 608, Phyllograptus typus ; 609, the supposed young of a Graptolite. Hall. 



other genera. They occur either in long, flat, notched threads spreading from 

 a center (Fig. 604), or in simple forms; but most specimens are only frag- 



