THE JURASSIC PERIOD. 238 
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 (Ophzurotds) are abundant 
in the Jurassic rocks, and the Sea-urchins (ZcAznoids) 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 ZcAznozds 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. 
Vig. 163.—Hemicidaris crenularis, showing the great tubercles on which the 
spines were supported. Middle Oolites. 
Passing over the Azneides, which, though not uncommon, 
are of little special interest, we come to the A7teculates, which 
also require little notice. Amongst the Crustaceans, whilst the 
little Water-fleas (Ostvacoda) 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 (Brachyura) 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 4rticulates, we meet in the Oolitic 
rocks with the remains of Spiders (Avachnida), Centipedes 
(Myriapoda), and numerous true Insects (/vsecfa). 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 Paleontina Oolitica of the Stonesfield slate—the rela- 
