20 



NORTH AMERICAN INDEX FOSSILS. 



Phylum III. CCELENTERATA. • 



Class Hydrozoa Huxley. 



This class comprises the modern hydroids, delicate moss-like 

 animals, mostly growing in branching clusters or colonies having a 

 common stem or a common base. A 

 few simple forms occur in fresh wa- 

 ter (Hydra), but with only one ex- 

 ception, the branching and clustered 

 species are all marine. They are 

 commonly found attached to the sea- 

 weed fringing our rocky shores, or 

 growing in the deeper waters, though 

 many also grow on rocks, where they 

 are attached by a network of delicate 

 root-like stolons, while others again 

 are attached to shells or live partly 

 buried in the mud. 



Hydroids of the modern type are 

 rare in the fossil state, but two aber- 

 rant groups the Graptolites and the 

 Hydrocorallines are abundantly repre- 

 sented in American strata. It will 

 be necessary, however, to briefly consider the characters of a mod- 

 ern hydrozoan in order to understand 

 the essential characteristics of the ex- 

 tinct species. For this purpose, Scrtu- 

 laria piunila (Fig. 31), a type abund- 

 ant on the rock-weed all' along the 

 north Atlantic coast, will be selected, 

 since in its general form it suggests the 

 characteristics of the Graptolites, though 

 in reality quite distinct from them. 



In general appearance this species 

 resembles a delicate plant, consisting of 

 Fig. 31. Sertuiaria pumila branching stems furnished with a double 

 Recent (after Nutting), p, polyp ; row f opposite teeth, like those of a 

 gon l saw. Under the microscope it is seen 

 that the organism consists of two es- 

 sential parts, easily distinguished if the specimen is properly stained 



Fig. 30. Ischadites iowensis 

 (Minn. Geol. Surv. ). 



ht, hydrotheca 

 (much enlarged) 



