THE JURASSIC PERIOD. 233 



foreign body by means of a jointed stem, from which they 

 detach themselves when fully grown to lead an independent 

 existence. In this later stage of their life, therefore, they 

 closely resemble the Brittle-stars in appearance. True Star- 

 fishes (Asteroids] and Brittle-stars (Ophiuroids) are abundant 

 in the Jurassic rocks, and the Sea-urchins {iLchinoids) are so 

 numerous and so well preserved as to constitute quite a marked 

 feature of some beds of the series. All the Oolitic urchins 

 agree with the modern EcJunoids in having the shell composed 

 of no more than twenty rows of plates. Many different genera 

 are known, and a characteristic species of the Middle Oolites 

 (Hemicidaris crenularis, fig. 163) is here figured. 



Fig. 163. Hsmicidat 



spines we 



Passing over the Annclides, which, though not uncommon, 

 are of little special interest, we come to the Articulates, which 

 also require little notice. Amongst the Crustaceans^ whilst the 

 little Water-fleas (Ostracoda) are still abundant, the most mark- 

 ed feature is the predominance which is now assumed by the 

 Decapods the highest of the known groups of the class. True 

 Crabs (Brachynrd) are by no means unknown ; but the prin- 

 cipal Oolitic Decapods belonged to the " Long-tailed " group 

 (Macrura), of which the existing Lobsters, Prawns, and 

 Shrimps are members. The fine-grained lithographic slates of 

 Solenhofen are especially famous as a depot for the remains 

 of these Crustaceans, and a characteristic species from this 

 locality (Eryon arctiformis, fig. 164) is here represented. 

 Amongst the air-breathing Articulates, we meet in the Oolitic 

 rocks with the remains of Spiders (Arachnida), Centipedes 

 (Myriapoda), and numerous true Insects (Insecta). In con- 

 nection with the last-mentioned of these groups, it is of interest 

 to note the occurrence of the oldest known fossil Butterfly 

 the Palceontina Oclitica of the Stonesfield slate the rela- 



