464 THE JURASSIC PERIOD 



The Echinodermata, especially the Crinoids and Sea-urchins, 

 are of great importance. The Crinoids are vastly more abundant 

 than they had been in the Trias, and although the number of 

 genera and species is not at all comparable to the great assem- 

 blage of Carboniferous time, yet for profusion and size of indi- 

 viduals the Jurassic has never been surpassed. Especially charac- 

 teristic are the superb species of Pentacrinus (PI. IX, Fig. 1), a 

 genus which still exists in the West Indian seas. Other common 

 genera are Apiocrinus and Eugeniacrinus. These genera all 

 belong to the Neocrinoidea, which have a very different type of 

 structure from the Palaeozoic forms, but, like nearly all the latter, 

 they were attached to the sea-bottom by their long stems. In the 

 Jurassic first appear the free-swimming Crinoids, like Comatula, 

 the commonest of modern genera. These animals possess a stem 

 only in their early stages of development ; subsequently they 

 become detached and free. Star-fishes and Brittle Stars are not 

 very common, but have attained a completely modern structure. 



The Echinoids have undergone a wonderful expansion and 

 diversification by the time of the Middle Jurassic. In the Lias, as 

 in the Trias, we find only the regular, radially symmetrical sea- 

 urchins, with mouth and anus at the opposite poles of the shell, 

 but in the middle and upper Jura appear the irregular Spatangoids 

 and Clypeastroids. In these the shell is bilaterally symmetrical, 

 rather than radially so, the anus, and even the mouth, losing their 

 polar positions, and the shape of the ambulacral areas being 

 greatly changed. This is another instance of the attainment of 

 modern structure which so many of the Mesozoic Invertebrates 

 display. 



Arthropoda. — The Crustacea are not found in very many locali- 

 ties, but places like the famous lithographic limestones of Solen- 

 hofen in Bavaria, where the conditions of preservation were 

 favourable, show that this group was very abundant and far ad- 

 vanced in the Jurassic seas. The long-tailed (macrurous) Deca- 

 pods (of which the lobster is a familiar example) are in' the 

 ascendant and are represented by many genera, several of which 

 still exist. The Crabs, or short-tailed Decapods, which are now 



