PALEOZOIC ROCK IS AX I) ERA. 



277 



characteristic of Silurian, and any one may identify Silu- 

 rian by means of them (Figs. 179, 180). 



FIG. 179. FIG. 180. 



FIGS. 179, 180. Silurian hydrozoa : 179. Diplograptus prietis. (After Nicholson.) 

 180. Graptolites Clintonensis. (After Hall.) 



Ecliinoderms ; Crinoids. At the present time, if we 

 leave out sea-cucumbers (Holo- 

 thurians), because, having no 

 shells, they are not preserved as 

 fossils, Echinoderms are of three 

 orders : 1. Echinoids, or sea-ur- 

 chins ; 2. Asteroids, or star-fishes ; 

 and, 3. Crinoids. The first two 

 are free-moving, the last is 

 stemmed. The first two are now 

 very abundant, the last rare. 

 But in Silurian times it was the 

 reverse. The Echinoids did not 

 exist at all, the Asteroids were 

 rare, but the Crinoids extremely 

 abundant, though, of course, of FlG - 181 - Livin e 



.. , ,, ,., Pentacrinus caput-medusae. 



species and genera wholly differ- 

 ent from any now existing (Fig. 181). It is well to ob- 

 serve that the crinoid is a lower form than the other two, 



