PALEOZOIC ROCKS AND ERA, 



279 



the constantly increasing free forms. But are there any 

 characters by which we may easily recognize those pecu- 



-PALEOZOIC 

 SILURIAN. DEVON" CAPBON1F* 



STEMMED 

 FIG. 185. Diagram showing uistiibution in ti;ne of crinoidg. 



liar to the Silurian ? There are. Crinoids are subdi- 

 vided into three main groups, viz. : 1. Crinids, or 

 plumose-armed crinoids (Fig. 182) ; 2. Blastids (Fig. 

 242, page 316), or bud-crinoids ; 3. Cystids (Figs. 183, 

 184), or bladder-crinoids. The crinids are not character- 

 istic of Silurian, nor even of Paleozoic ; the blastids are 

 characteristic of Paleozoic, though not of Silurian ; the 

 cystids are characteristic of Silurian alone. This is rep- 

 resented by subdivisions of the shaded space in Fig. 185, 

 in relation with the subdivision of the Paleozoic. 



Mollusks ; Brachiopods. Bivalve shells are divided 

 into two great groups, viz. : 1. Common bivalves (Lamel- 

 libranchs) ; and, 2. Lamp-shells, or Brachiopods. At 

 present, the former are extremely abundant, and the lat- 

 ter rare. The reverse was true in Silurian times. The 

 distribution in time of the two kinds may be roughly 



A 



FIG. 186. Diagram showing the general distribution, in time, of brachiopods and 



lamellibranchs. 



represented by the diagram (Fig. 186). Now. brachio- 

 pods are very different from, and much lower than, ordi- 

 nary bivalves. Lamellibranchs have a right and left 

 valve right and left gills, etc. ; in brachiopods the 



