SILURIAN PERIOD 31 



central discs were still in being. Certain forms were now 

 more in line with those of to-day in having a thorough 

 protection for their tube-feet (Lepidaster). Some of the BRITTLE- 

 brittle-stars had undergone a similar modification (Lapwor- STARS 

 thura). The latter animals were now well distinct from 

 the starfishes ; and by coiling round crinoids and other 

 acrobatic feats they doubtless displayed their superior 

 agility. 



Various forms of sea-urchins were groping about amongst SEA-URCHINS 

 the seaweeds, or lurking in rock-hollows. They all differed 

 from the known Ordovician forms (which had become 

 extinct) in having tests with more numerous vertical plate- 

 rows. In fact the plate-rows were more numerous than those 

 on the body-coverings of any urchins now living. The tests 

 of some were of a quite primitive character, the plates being 

 thin and irregularly arranged (Echinocystis}. In a few cases 

 the under-part of the test was not perforated, and the lower 

 tube- feet had to be protruded between the plates (Palceo- 

 discus}. The tests, no doubt, were highly flexible ; and a 

 very slight misfortune would have cracked them across 

 had they been otherwise. Urchins thus encased, however, 

 were not the best clad of the time ; for certain forms were 

 abroad in tests of stout, rigidly attached, and well-perforated 

 plates (Palczechinus). These animals were about the size of 

 greengages, and were more or less covered with short bristles. 

 They possessed, moreover unlike their humble brethren a 

 well-developed jaw apparatus, capable of gnawing into the 

 shells of molluscs. They were certainly the " fittest " urchins 

 in the seas, and proved fit enough, indeed, to continue their 

 line through the next two Periods. 



The Ordovician worms, that had betaken themselves to WORMS 

 extremely sedentary habits in chalky tubes cemented to rocks, 

 had by no means entered on a fatal course. In spite of aggres- 

 sive sea-urchins, and other intruders on their privacy, 

 numerous descendants were living in this Period. Indeed, the 

 line has been perpetuated until the present time (Serpulites). 



Brachiopods or lampshells were in a state of vigour they LAMPSHELLS 

 had never experienced before, and were never to know 

 again. Not only on the coasts of Europe, Asia, and North and 



