39° 



THE SILURIAN PERIOD 



Echinodermata. — In this group we observe a diminution of the 

 Cystidea, but a marked increase of the Crinoids ; Eucalyptocrinus 

 (see Fig. 139/1) is a good example. Star-fishes also have grown 

 more abundant. A new class of the Echinoderms now makes its 

 first appearance, the Blastoidea. This class is extinct at present and 



Silurian Fossils 



Fig. 139. — 1. Eucalyptocrinus crassus, 1/2. 2. Dalmanites limulurus. 3. Lichas 

 Boltoni, 1/3. (After Hall.) 



its structure is not well understood ; the group remains rare in the 

 Silurian and Devonian, first becoming important in the Carbonifer- 

 ous. The Echinoids, or sea-urchins, which were commoner than 

 before, have no arms, but a closed spheroidal or discoidal test, made 

 up of calcareous plates, which in all the modern sea-urchins are 

 arranged in just twenty vertical rows, and are closely fitted to- 

 gether by their edges, like a mosaic pavement. In the Palaeozoic 



