from or referring to the period of time between about 445 and 420 million years ago, in which fish w

中文解释

志留纪的（迄今约4亿4500万至4亿2000万年前的地质时间段，此时期有颌骨的鱼类以及陆地上的植物和小型动物第一次出现） The grey slates and shales of the Silurian rock are obvious as you walk.