308 



PALAEOZOIC SYSTEM OF ROCKS. 



Hydrozoa. — The perfect forms of this class, viz., Medusae, or jelly- 

 fishes, are so soft and perishable that, with one or two exceptions in 



the Mesozoic rocks, 

 they are not found pre- 

 served at all in the 

 strata of any geological 

 period. They may or 

 may not have existed 

 at this time ; probably 

 they did not. But the 

 larval form of most, if 

 not all, Medusae is a 

 compound polypoid an- 

 imal, forming a minute- 

 ly-branching, horny, or 

 coralline axis. These 

 ^^ minutely - branching 

 axes are strung on each 

 side with cells, in which 

 are inclosed little poly- 

 poid animals. They 

 grow in still, quiet wa- 

 ters, and are often mis- 

 taken by the unscien- 

 tific for sea - weed. 

 These, by their compo- 

 sition, are well adapted 

 for preservation, and it 

 is this larval form, 

 therefore, only that we 

 might expect to find. 

 Figs. 309-311 are ex- 

 amples of living forms. 

 Xow, in very fine 

 shales of Silurian age, 

 especially of Lower Si- 

 lurian and Cambrian, 

 are found abundantly 

 beautiful impressions of 

 an organism which is 

 most probably a com- 

 Dipiograptu^ pristis (after pound Hydrozoan al- 

 lied to Sertularia of the 



Fig. 313. 



Fig. 314. 



Fig. 316. 

 Figs. 313-317.— Graptolites 



313. 



Nicholson). 314. Phylloeraptus typus (after Hall). 315. Di- 



dvniograptus V-fractus (after Hall). 316. Graptolithus Logani 



(after Hall). 317. Monograptus priodon: a. side view; b, r . VPQP11 4- /q flv TliPV HV9 



back view: c, front view, showing opening (after Nicholson), pit^em ua) . ±uvy aic 



